aria after aria "passes through our mind." Such experiences we describe by saying that an external stimulus arouses us to a response or starts associations of ideas.

The response to a stimulus may be "impulsive” like the jump when a fire-cracker goes off. It is "deliberate" when we take our time about it. The reaction in the first case is a simple one including perception and volition (p. 30), in the other a complex one including discrimination and choice (p. 40).

For most purposes we may assume that the association of ideas (p. 41) proceeds according to the rule of coincidence; an idea is followed by one that has something in common with it. This common factor may be some component; e. g., the word "street" may be followed by the thought of the word "strong," both beginning with the same letters "str." Or the com mon factor may be the larger idea of which both ideas are part; e. g. "street" may be followed by "car," because both have occurred as parts of the idea "street car."

This rule does not explain why on one occasion strong" is associated and on another occasion "car." My explanation of the association of ideas is as follows:

Every idea in the mind fades away at first rapidly, then more and more slowly (see the memory curve p. 209); although it soon becomes so faint that we no longer notice it, it never entirely disappears; its intensity approaches zero asymptotically, as the mathematicians say. Whenever the same idea enters the mind again, it is strengthened by what remains of the first one. This may occur any number of times, the idea

gaining each time. It is a familiar experience with all of us that the oftener we see a thing the more often it comes into the mind at other times.

This law holds good of each element of the ideas; like elements fuse although they may be in different ideas. For example, the elements "s-t-r" fuse together when such a series of words as "struggle, strive, strict, straw, strong," etc, is repeated. Now let a new idea enter, say "street"; the elements "str" fuse with the same ones of the series of words and this makes them so strong that they determine the association. This is why "strong" is associated and not car." We can carry the same principle further and explain why "strong" and not "strive" was associated. These principles the reader can investigate for himself. He can have a person read over many times a long list of words; then on calling out to him some word, he will be able to notice the influence of the list on the associations.

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There are thus three fundamental laws of association of ideas: 1, each element of an idea persists in an intensity that steadily decreases toward but never reaches zero; 2, every element of an idea adds its intensity to the residual intensity of every preceding element of the same kind; 3, an element adds to the intensity of any other element with which it was previously connected.

During our waking hours our associations of ideas are continually started anew by impressions from the world about us; the persons talking to us, the sights we see, the sounds we hear, all force us to think along certain lines. In revery we are left more to our

selves and our thoughts have freer play. In sleep the impressions from outside are few and weak; we dream. long continued stories which, freed from outside influences, may appear quite absurd when we awake. Yet, although our thoughts are freest in dreams, they are still influenced by impressions from the skin, intestines, etc.

"Is mind governed by law?" By "law" we mean an established sequence of events. If we let go of a box, it falls to the ground; this we say is an illustration of the law of gravitation. If the box does not fall, we may do one of two things. We may say: "Here is a case that does not conform to law; therefore we must admit the existence of mysterious forces concerning which we are at liberty to believe anything we please." Such reasoning produces the "mystics," whose fundamental principle is, that, since there are things we cannot explain by laws, therefore belief is at liberty to set up any laws it pleases. This is the basis of clairvoyance, spiritualism, thought-transference, telepathy, palmistry, and similar delusions.

When the box does not fall, the common-sense man says: "The case apparently does not conform to law, but let us inquire if some unseen or undiscovered force is not counteracting gravitation. Even if I myself cannot find it, yet I believe only in the action of forces according to laws and I will search for the hidden one or leave more able men to do so." On investigating the box he finds an unnoticed string that holds it up, or some similar arrangement that had escaped him. Men of this kind-whether trained or not-are men of scientific habits of thought. The advance of

science is one continuous battle of the scientists against

the mystics.

Applying the principles of science to mental life, we establish the laws of reaction, of habit, etc. Where these laws do not suffice, we simply say so and wait for further information; we absolutely reject all mystic explanations. Mind, therefore, we assume to be governed by law even to its finest details.

CHAPTER XXI

MATERIALISM AND SPIRITUALISM IN PSYCHOLOGY

the good old days, now happily gone forever, when psychology was a matter of doctrine, we used to hear of materialistic psychology, spiritualistic psychology, the psychology of Hamilton, the psychology of Hegel, English psychology, German psychology, etc., etc.

Nowadays it is just as absurd to speak of anybody's system of psychology as to speak of anybody's system of chemistry. There is one science of chemistry to which all scientific chemists are contributors; there is one science of psychology which all scientific psychologists make their humble efforts to develop. How this has come about I am going to tell by translating a few pages from Wundt's Vorlesungen über Menschen und Thierseele.

"The earliest psychology is materialism. The soul is air or fire or an ether; it remains, however, material nothwithstanding the efforts to lighten and thereby to spiritualise the matter. Among the Greeks it was Plato who first freed the soul from the body, whereby he made it the ruling principle of the latter. He thus opened the path for the one-sided dualism which regarded sensory existence as the contamination and degradation of a purely mental being. Aristotle, who