A fact perceived for the first time must needs be attended to, in order that it may be perceived. That is, the first and original perception of a fact is a highly conscious response. But the perception of a fact, like any other form of response, becomes easy with practice; the linkage of stimulus and response becomes stronger and stronger, till finally the stimulus arouses the perceptive response almost automatically. The familiar fact is perceived without receiving close attention, or even without receiving any attention.

While your attention is absorbed in reading or thinking, you may respond to the sight of the flower in a vase on your table by knowing it to be there, you may respond to the noise of the pa.s.sing street car by knowing what that is, and you may respond to the contact of your foot with the leg of the chair by dimly knowing what that object is. A great deal of this inattentive perception of familiar facts is always going on. Aside from sensation and from some of the reflexes, the perception of familiar facts is the most practised and the easiest of all responses.

The laws and sub-laws of learning apply perfectly to practised perception. The more frequently, the more recently, and the more intensely a given fact has been perceived, the more readily is it perceived again. The more a given fact is in line with the mental set of the moment, the more readily is it perceived. Sometimes it is so readily perceived that we think we see it when it isn't there. If you are hunting for a lost knife, anything remotely resembling {434} a knife will catch your eye and for an instant be perceived as the missing object.

The principle of _subst.i.tute stimulus_ applies remarkably well to practised perception. The first time you perceive an object, you observe it attentively, and expose your perceptive apparatus to the whole collection of stimuli that the object sends your way. The next time you need not observe it so attentively, for you make the same perceptive response to a _part_ of the original collection of stimuli.

The response originally aroused by the whole collection of stimuli is later aroused by a fraction of this collection. The stimulus may be _reduced_ considerably, and still arouse the perception of the same fact. A child is making the acquaintance of the dog. The dog barks, and the child watches the performance. He not only sees the dog, and hears the noise, but he _sees_ the dog _bark_, and _hears_ the dog _bark_. This original perception is a unitary response to the combination of sight and sound. Thereafter he does not require both stimuli at once, but, when he hears this noise, he perceives the dog barking, and when he sees the dog he sees an object that can bark. In the same way, a thousand objects which furnish stimuli to more than one of the senses are perceived as units, and, later, need only act on a single sense to be known.

The stimulus, instead of being reduced, may be _modified_, and still arouse the same perception as before. A face appears in the baby's field of view, but away across the room so that it is a very small object, visually. The face approaches and gradually becomes a larger visual object, and the light and shadow upon it change from moment to moment, but it remains nearly enough the same to arouse essentially the same perception in the child. He comes to know the face at various distances and angles and under various lights.

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Again, the child holds a block in his hands, and looks at it square on, so that it is really a rectangle in his field of view. He turns it slightly, and now it is no longer visually a rectangle, but an oblique parallelogram. But the change is not enough to abolish the first perception; he sees it as the same object as before. By dint of many such experiences, we see a book cover or a door as a rectangle, no matter at what angle we may view it, and we know a circle for a circle even though at most angles it is really an ellipse in the field of view. A large share of practised perceptions belong under the head of "response by a.n.a.logy",[Footnote: See p. 406.] since they consist in making the same response to the present stimulus that has previously been made to a similar but not identical stimulus. If every modified stimulus gave a new and different perception, it would be a slow job getting acquainted with the world. A thing is never twice the same, as a collection of stimuli, and yet, within wide limits, it is always perceived as the same thing.

Corrected Perception

Response by a.n.a.logy, however, often leads us astray, in making us perceive a new object as essentially the same as something already familiar. First impressions of a new object or acquaintance often need revision, because they do not work well. They do not work well because they are rough and ready, taking the object in the lump, with scant attention to details which may prove to be important. It is easy to follow the law of combination and respond to a whole collection of stimuli, but to break up the collection and isolate out of it a smaller collection to respond to--that is something we will not do unless forced to it. Isolation and discrimination are uphill work.

When they occur, it is {436} because the rough and ready response has proved unsatisfactory,

_Subst.i.tute response_ is the big factor in corrected perception, as subst.i.tute stimulus is in practised perception. When our first perception of an object gets us into difficulties, then we are forced to attend more closely and find something in the object that can serve as the stimulus to a better response. This is the process by which we isolate, a.n.a.lyze, discriminate.

Our old friend, the white rat, learned to enter a door only if it bore a yellow sign. [Footnote: See p. 304.] It was uphill work for him, hundreds of trials being required before the discriminating response was established; but he learned it finally. At the outset, a door was a door to the rat, and responded to as such, without regard to the sign. Whenever he entered a door without the sign, he got a shock, and scurried back; and before venturing again he looked all around, seeking, we may say, a stimulus to guide him; incidentally, he looked at the yellow disk, and this stimulus, though inconspicuous and feeble to a rat, finally got linked up with the entering response. The response of first finding and then following the sign had been subst.i.tuted for the original response of simply entering.

In the same way the newly hatched chick, which at first pecks at all small objects, caterpillars included, learns to discriminate against caterpillars. In a practical sense, the chick, like the rat, learns to distinguish between stimuli that at first aroused the same response.

It is in the same way that the human being is driven to discriminate and attend to details. He is brought to a halt by the poor results of his first rough and ready perception, scans the situation, isolates some detail and, finding response to this detail to bring satisfactory results, subst.i.tutes response to this {437} detail for his first undiscriminating response to the whole object.

The child at first treats gloves as alike, whether rights or lefts, but thus gets into trouble, and is driven to look at them more sharply till he perceives the special characteristics of rights and lefts. He could not describe the difference, to be sure, but he sees it well enough for his purposes. If you ask an older person to describe this difference, and rally him on his inability to do so, he is thus driven to lay them side by side and study out the difference still more precisely.

The average non-mechanical person, on acquiring an automobile, takes it as a gift of the G.o.ds, a big total thing, simply to sit in and go.

He soon learns certain parts that he must deal with, but most of the works remain a mystery to him. Then something goes wrong, and he gets out to look. "What do you suppose this thing is here? I never noticed it before". Tire trouble teaches him about wheels, engine trouble leads him to know the engine, ignition trouble may lead him to notice certain wires and binding-posts that were too inconspicuous at first to attract his attention. A car becomes to him a thing with a hundred well-known parts, instead of just one big totality.

Blocked response, closer examination, new stimulus isolated that gives satisfactory response--such is, typically, the process of a.n.a.lytic perception.

Sensory Data Serving as Signs of Various Sorts of Fact

Among facts perceived, we may list things and events, and their qualities and relations. Under "things" we here include persons and animals and everything that would ordinarily be called an "object".

Under "events", we include movement, change and happenings of all sorts. Under {438} "qualities" we may include everything that can be discovered in a thing or event taken by itself, and under "relations"

anything that can be discovered by comparing or contrasting two things or events. The "groups" that we have several times spoken of as being observed would here be included under "things"; but the strict logic of the whole cla.s.sification is not a matter of importance, as the only object in view is to call attention to the great variety of facts that are perceived.

Now the question arises, by what signs or indications these various facts are perceived. Often, as we have seen, the fact is by no means fully presented to the senses, and often it is far from easy for the perceiver to tell on what signs the perception depends. He knows the fact, but how he knows it he cannot tell. A large part of the very extensive experimental investigation of perception has been concerned with this problem of ferreting out the signs on which the various perceptions are based, the precise stimuli to which the perceptions respond.

For example, we can examine objects by feeling of them with a stick held in the hand, and thus perceive their roughness or smoothness; but how do we sense these facts? It seems to us as if we felt them with the end of the stick, but that is absurd, since there are no sense organs in the stick. It must be that we perceive the roughness by means of sensations arising in the hand and arm, but to identify these sensations is a much harder task than to discover the objective fact of roughness.

Again, we distinguish the tones of two musical instruments by aid of their overtones, but elaborate experiments were required to prove this, since ordinarily we do not distinguish the overtones, and could simply say that the instruments sounded differently, and let it go at that.

Once more, consider our ability to perceive time intervals; {439} and to distinguish an interval of a second from one of a second and a quarter. How in the world can any one perceive time? Time is no force that could conceivably act as a stimulus to a sense organ. It must be some change or process that is the stimulus and that serves as the indication of duration. Most likely, it is some muscular or internal bodily change, but none of the more precise suggestions that have been offered square with all the facts. It cannot be the movements of breathing that give us our perception of time, for we can hold our breath and still distinguish one short interval from another. It cannot be the heart beat, for we can beat time in a rhythm that cuts across the rate of the heart beat. When a singer is accompanying himself on the piano, keeping good time in spite of the fact that the notes are uneven in length, and meanwhile using his feet on the pedals, what has he got left to beat time with? No one has located the stimulus to which accurate time perception responds, though, in a general way, we are pretty sure that change of one sort or another is the datum. With longer intervals, from a minute to several hours, the sign of duration is probably the amount happening in the interval, or else such progressive bodily changes as hunger and fatigue.

The Perception of s.p.a.ce

Stimuli for the perception of location are provided by all the senses.

We perceive a taste as in the mouth, thirst as in the throat, hunger pangs as in the stomach. To a familiar odor we may respond by knowing the odorous substance to be close at hand. To stimulation of the semi-circular ca.n.a.ls we respond by knowing the direction in which we are being turned.

We respond to sounds by knowing the direction from which they come, and the distance from which they come; {440} but it must be confessed that we are liable to gross errors here. To perceive the distance of the sounding body we have to be familiar with the sound at various distances, and our perception of distance is based on this knowledge.

As to the direction of sound, experiment has proved that we do little more than distinguish between right and left; we are all at sea in attempting to distinguish front from back or up from down. Apparently the only datum we have to go by is the different stimulation given the two ears according as the sound comes from the right or left.

The remaining senses, the cutaneous, the kinesthetic and the visual, afford much fuller data for the perception of spatial facts. Movements of the limbs are perceived quite accurately as to direction and extent.

A cutaneous stimulus is located with fair exactness, though much less exactly on such regions as the back than on the hands or lips. If you were asked how you distinguished one point from another on the back of the hand, you could only answer that they felt different; and if you were further asked whether a pencil point applied to the two points of the skin did not feel the same, you would have to acknowledge that it did feel the same, except that it was felt in a different place. In other words, you would not be able to identify the exact data on which your perception of cutaneous position is based. Science has done no better, but has simply given the name of "local sign" to the una.n.a.lyzed sensory datum that gives a knowledge of the point stimulated.

In handling an object, as also in walking and many other movements, the cutaneous and kinesthetic senses are stimulated together, and between them furnish data for the perception of many spatial facts, such as the shape of an object examined by the hand. The spherical shape is certainly better perceived by this combination of tactile and kinesthetic {441} sensations than by vision, and the same is probably true of many similar spatial facts. That is, when we see a round ball, the visual stimulus is a subst.i.tute for the tactile and cutaneous stimuli that originally had most to do with arousing this perception.

In part by this route of the subst.i.tute stimulus, the sense of vision comes to arouse almost all sorts of spatial perceptions. Of itself, the retina has "local sign" since we can tell where in the field of view a seen object is, i.e., in what direction it is from us. This visual perception of location is so much more exact than the cutaneous or kinesthetic that it cannot possibly be derived from them; and the same is true of the visual perception of difference in length, which is one of the most accurate forms of perception. The retina must of itself afford very complete stimuli for the perception of location and size, as far as these are confined to the two dimensions, up-down and right-left. But, when you stop to think, it seems impossible that the retina should afford any data for perceiving distance in the front-back dimension.

The retina is a screen, and the stimulus that it gets from the world outside is like a picture cast upon a screen. The picture has the right-left and up-down dimensions, but no front-back dimension. How, then, does it come about, as it certainly does, that we perceive by aid of the eye the distance of objects from us, and the solidity and relief of objects? This problem in visual perception has received much attention and been carried to a satisfactory solution.

Consider, first, what stimuli indicative of distance and relief could affect a single motionless eye. The picture on the retina could then be duplicated by a painter on canvas, and the signs of distance available would be the same in the two cases. The painter uses foreshortening, making a man in the picture small in proportion to his distance away; {442} and in the same way, when any familiar object casts a small picture on the retina, we perceive the object, not as diminished in size, but as far away. The painter colors his near hills green, his distant ones blue, and washes out all detail in the latter--"aerial perspective", he calls this. His distant hill peeks from behind his nearer one, being partially covered by it. His shadows fall in a way to indicate the relief of the landscape. These signs of distance also affect the single resting eye and are responded to by appropriate spatial perceptions.

Now let the single eye move, with the head, from side to side: an index of the distance of objects is thus obtained, additional to all the painter has at his disposal, for the distant objects in the field of view now seem to move with the eye, while the nearer objects slide in the opposite direction. How much this sign is ordinarily made use of in perceiving distance is not known; it is believed not to be used very much, and yet it is the most delicate of all the signs of distance. The reason why it may not be much used by two-eyed people is that another index almost as delicate and handier to use is afforded by binocular vision.

When both eyes are open, we have a sign of distance that the painter does not use, though it is used in stereoscope slides. The right and left eyes get somewhat different views of the same solid object, the right eye seeing a little further around the object to the right, and the left eye to the left. The disparity between the two retinal images, due to the different angles at which they view the object, is greatest when the object is close at hand, and diminishes to practically zero when it is a few hundred feet away. This disparity between the two retinal images is responded to by perception of the distance and relief of the object.

It will be recalled [Footnote: See pp. 253-254.] that when two utterly inconsistent {443} views are presented to the two eyes, as a red field to one and a green field to the other, the visual apparatus balks and refuses to see more than one at a time--the binocular rivalry phenomenon. But when the disparate views are such as are presented to the two eyes by the same solid object, the visual apparatus (following the law of combination) responds to the double stimulation by getting a single view of an object in three dimensions.

Esthetic Perception

Beauty, humor, pathos and sublimity can be perceived by the senses, though we might debate a long time over the question whether these characteristics are really objective, or merely our own feelings aroused by the objects, and then projected into them. However that may be, there is no doubt that the ability to make these responses is something that can be trained, and that some people are blind and deaf to beauty and humor that other people clearly perceive. Many a one fails to see the point of a joke, or is unable to find any humor in the situation, which are clearly perceived by another. Many a one sees only a sign of rain in a great bank of clouds, only a weary climb in the looming mountain.

"A primrose by the river's brim A yellow primrose was to him.

And it was nothing more."

It would not be quite fair to describe such a one as lacking in feeling; he probably has, on sufficient stimulus, the same feelings as another man, and it would be more exact to say that he is lacking in perception of certain qualities and relations. He probably tends, by nature and training, to practical rather than esthetic perception. To see any {444} beauty in a new style of music or painting, or to sense the humor in a new form of humorous writing, you need to be initiated, to be trained in observing the precise qualities and relations that are depended on for the esthetic effect. A complex situation presents almost an unlimited range of facts that may be perceived; no one perceives them all, and which he shall perceive depends on his nature and training, as well as on his att.i.tude or mental set at the moment when the situation is presented.

Psychology has not by any means been idle in this field of esthetics; it has developed experimental methods for determining the preferences of individuals and of social groups. But it must be confessed that the results offer little that can be succinctly summarized.

One curious result is that even the very simplest objects can produce an esthetic effect. You would scarcely suppose, for example, that a mere rectangle could produce any esthetic effect, or that it would make any difference what exact proportions the rectangle possessed; and yet it is found that some rectangles are preferred to others, and that the popular choice falls upon what the art theorists have long known as the "golden section", a rectangle with a width about sixty-two per cent, of its length. Also, however much you may like symmetry, you would scarcely suppose that it could make much difference where, on a horizontal line, a little cross line should be erected; and yet nearly every one, on being tested, will agree that the middle is the best point. These are merely a couple of sample results from the numerous studies in this field.

Social Perception

By the senses we perceive the motives and intentions of other people, their sincerity, goodness, intelligence, and {445} many other traits.

We see them angry or bored, amused, full of energy. To be sure, none of these human characteristics is directly and fully sensed, but that is the case also with many characteristics of inanimate objects which, nevertheless, we perceive by aid of the senses. We perceive anger or sincerity in much the same way that we perceive moisture or smoothness by the eye. To experience the anger of another person is a complex experience, but a single element from this experience may come to serve as the sign of the whole condition. A good share of the child's undirected education consists in learning to perceive the intentions and characteristics of other people by aid of little signs. He learns to read the signs of the weather in the family circle, and he learns in some measure to be a judge of men.

I once saw an instructive little incident, in which an older boy suddenly grabbed the cap from a little boy's head, and held it out to the driver of a pa.s.sing automobile, as if giving it to him. The man saw the joke, and drove on laughing, but the little boy took it seriously and was quite worried for fear the man would carry off his cap. An older child would have "seen into" the situation readily; he could not have been teased in that way. Many social situations which are "all Greek" to a little child are understood readily by an older person.

It would be very valuable if psychology could succeed in a.n.a.lyzing out the signs by which such a trait as intelligence or "will power" is perceived, so as to reduce such perception to a science; but it is very doubtful if this can be done. Some persons who probably have themselves a keen perception of such traits have put forward systems, based upon the shape of the face, etc. They probably think they perceive human traits according to their systems, but the systems fail in other hands, and are undoubtedly {446} fallacious. No good judge of character really goes by the shape of the face; he goes by little behavior signs which he has not a.n.a.lyzed out, and therefore cannot explain to another person.