The concept of "contiguity" calls for further analysis in respect of the temporal relation between the contiguous items. The term co-presentation might, literally taken, suggest "simultaneous ap prehension"; but the question may be raised whether such simul taneity when it does occur is not, to a great extent, the fruit of succession. Items successively apprehended by the "movements" of attention come, through primary retention, to be co-present to the mind. We seem to see a picture all at once, but more precise observation would reveal a progressive apprehension in which what is prior in experience is retained until the object is seen as a whole. This is quite evident in hearing a spoken sentence.
Where two items are, in this sense, co-presented, they may be simultaneously revived, and each will revive the other with equal ease. Where, however, there is succession in presentation, there is strong presumptive evidence that associations are formed primarily in the "forward direction." That which follows in ex perience is revived more readily than that which precedes. Never theless there is also undeniable evidence that weaker associations are also formed in the reverse direction. Though we repeat the alphabet more easily in the order in which the letters were learnt, we can, if necessary, say it backwards too. This, however, may be due to primary retention.
In so far as that which is presented is retained, it is carried forward in experience side by side with "objectively" later im pressions, and may later become again the object of attention. In hearing a spoken sentence, the earlier words are in this way carried forward, and after attending to the meaning of the sentence as a whole we may note the mispronunciation of one of the earlier words. In which case, we may later associate with the memory of this statement the speaker's offending pronunciation rather than his subsequent conversation. In any case it is the order in experi ence rather than the order of stimulation which is psychologically important. And the order of experience is the order of attention. For this reason the law may be described as the "Law of Associa tion by Continuity of Attention." The Sequence of Ideas.—The course of experience is never wholly objectively determined nor wholly determined by subjec tive factors. Nevertheless, we may broadly say that perceptual process is predominantly determined by external circumstances whereas over the sequence of ideas we have a more direct control. The question arises how far does the law of association explain this latter type of sequence. As ordinarily stated the law of con
tiguity would suggest that the course of our ideas, whilst free from the domination of the present, is still in fetters to the sequences of the past. Except in so far as similarities provide, so to speak, a series of cross references, we find no escape from a merely historical order in our passages of thought. Obviously, however, the facts are against any such simple view. The law of association calls for considerable amplification.
In the first place a given item of experience occurs in a variety of contexts. Which of these is likely to be revived, and what of all that is similar will be selected by thought? To these questions a partial answer is provided by what we may call the sub-laws of association, the principles of Frequency, Recency and Intensity.
Given that the revival of any item in experience will tend to reinstate its context, the more frequently any given context has occurred the more probable that specific context will be the one revived.
Similarly, with regard to Recency: the more recently any spe cific context has accompanied a given item, the more likely that context is to be revived.
These two sub-laws admit of a fairly precise quantitative formu lation, the first being expressed by the "curve of learning," the second by the curve of oblivescence.
The "Law of Intensity" admits of a variety of interpretations, the term standing ambiguously for the intensive attribute of sensa tion, for intensity of feeling, or the "intensity" of attention. It is doubtful whether the mere intensity of sensation (apart from the effects upon feeling and attention) is of great importance. So far as it is, it occupies a still more subsidiary role among the very special laws which relate to the sensory attributes in general. Probably the best expression for the law of Intensity is as the law of Interest. The more interested the transition of attention from an item of experience to its context, the greater the tendency for the sequence to be revived. So stated, the formula is closely re lated to the so-called "Law of Effect," which latter refers, how ever, more particularly to the acquisition of skill, and to the formation of habit. In the association of movements much de pends on whether the action is followed by pleasure or pain. When the "effects" are pleasant the association is "stamped in," when painful it tends to be impaired.