James Mark Baldwin, answered from the texts and cited to the page.
Assimilation and accommodation are the two great complementary principles by which the organism — child or adult — meets the world and grows through meeting it. Neither is intelligible alone; they are the two faces of one adaptive process. Assimilation is the principle by which new experience is drawn into the channels of what is already established.
When a new presentation arrives, the organism does not encounter it in a vacuum; the old motor habits and memory-copies reach out, so to speak, and impose their pattern upon it.1 The energy of the new process finds itself drawn off in the discharge-channels of whatever old content it resembles, so that the novel is identified with the familiar, and recognition — and its errors, illusions — is the result.2
The new element is not simply added; it is unified with the old in a single, larger motor discharge that represents the habit of the organism so far built up.3 Accommodation is the opposing movement, and it is the principle of genuine growth. Where assimilation conserves and repeats, accommodation reaches forward: it is the principle by which an organism comes to adapt itself to more complex conditions of stimulation by performing more complex functions.4
Learning to act is just accommodation, nothing more nor less — speech, handwriting, piano-playing, every motor acquisition whatever.5 The mechanism is what I called the circular reaction: any reaction by which a useful stimulus is held onto and enjoyed, or a damaging one escaped, falls under this principle.6 Accommodation tends, by its very nature, to come into conflict with old habitual movements and so to disintegrate them — it runs ahead of habit, prospectively, where habit always looks back.7
The two work together, and neither can proceed without the other. Continued accommodation is possible only because habit all the time conserves the past and gives fixed points of support from which new accommodations can be projected.8 A child cannot learn the piano by plunging at once into complex performance; he must practise from the platform of the simple scales he already commands, assimilating the new to the established before he can accommodate to anything further.9
And on the side of truth — of knowledge that actually holds — a new experience must first pass the test of assimilation to established habits before it can be submitted to the harder test of accommodation to fact in the environment.10 What cannot be assimilated to the life of habit cannot come to be established as an intelligible fact at all; what passes that first test must still compete for survival in the domain of external reality.11
The seat of productive variation, from which genuine selections are possible, lies on the motor side — in the active life of the organism — because only there can internal relations be built up that genuinely reproduce external relations.12 Assimilation and accommodation are thus not merely psychological categories; they are the organism's two modes of being in a world it must both hold onto and surpass.
The external pattern is assimilated to the memory copy, or to the word or other symbol which comes to stand for it.Mental Development in the Child and the Race
The energy of the new presentation process finds itself drawn off in the channels of the discharge of the old one which it resembles; the motor associations, therefore, and with them all the organic and revived mental elements stirred up by them, come to identify or unite the new content with the old.Mental Development in the Child and the Race
the assimilation of any one element to another, or the assimilation of any two or more such elements to a third, is due to the unifying of their motor discharges in the single larger discharge which stands for the apperceived result.Mental Development in the Child and the Race
Accommodation is the principle by which an organism comes to adapt itself to more complex conditions of stimulation by performing more complex functions.Mental Development in the Child and the Race
Speech, tracery, handwriting, piano-playing, all motor acquisitions, are what accommodation is, i.e. adaptations to more complex conditions.Mental Development in the Child and the Race
any reaction by which a useful stimulus is hailed back and enjoyed, or a damaging one fled from and escaped — any such is a case of accommodation, and falls under the principle of 'circular reactions' or 'Imitation' now expounded.Mental Development in the Child and the Race
it tends, by the selection of new movements, to come into direct conflict with old habitual movements, and so to disintegrate habits.Mental Development in the Child and the Race
continued accommodation is possible only because the other principle, habit, all the time conserves the past and gives points d'appui in solidified structure for new accommodations.Mental Development in the Child and the Race
a child cannot play the piano, though he might actually go through a series of movements reproducing those of a skilled performer... But he can learn little by little, if he practise faithfully from the platform of the movements of the simple scales and finger exercises which he already knows how to perform.Development and Evolution
The first test, therefore, is that of assimilation to established habits. If we grant this, and also grant that subsequently to this there is a further selection, from such variations, of those which work in the environment, we get a double function of selection.Development and Evolution
things which are not assimilable to the life of habit cannot come to be established as intelligible facts. The great difference between the two tests is that that of habit is less exacting; for after a datum has passed the gantlet of habit... it must still compete for survival in the domain of fact.Development and Evolution
the seat of productive variations, of variations, that is, from which selections are possible, must be on the motor side, in the active life. Only thus could 'internal relations' be established which should be true to or should reproduce 'external relations.'Development and Evolution