The simplest intellectual work is to perceive physical objects.  Having perceived an object several times, the intellect lifts itself to a higher process, and knows it when it sees it again, remembers it. Perception is the first, the simplest, the initiatory intellectual process, memory is the second.  Higher than they, and rising out of them, is a third process, the one whereby are modified and transmuted the mental impressions of what is perceived or remembered.  A mother, just parted from her child, recalls his form and face, summons before her mind’s eye an image of him; and this image is modified by her feelings, she seeing him in attitudes and relations in which she had never seen him before, cheerful or sad according to her mood.  This she could not do by aid of memory alone; she could not vary the impress of her boy left on the brain; she could not vividly reproduce it in shifting, rapidly successive conditions; she could not modify and diversify that impress; in a word, she could not liberate it.  Memory could only re-give her, with single, passive fidelity, what she had seen, unmodified, motionless, unenlivened, like a picture of her boy on canvas.  Urge intellectual activity to the phase above memory, and the mental image steps out from its immobility, becomes a changeful, elastic figure, brightened or darkened by the lights and shadows cast by the feelings; the intellect, quick now with plastic power, varying the image in position and expression, obedient to the demands of the feelings, of which it is ever the ready instrument.  This third process is imagination.

Through this mode of intellectual action the materials gathered in the mind are endlessly combined and modified.  In all intellectual activity, beyond bare perception and memory, imagination in some degree is and must be present.  It is in fact the mind handling its materials, and in no sphere, above the simplest, can the mind move without this power of firmly holding and molding facts and relations, phenomena and interior promptings and suggestions.  To the forensic reasoner, to the practical master-worker in whatever sphere, such a power is essential not less than to the ideal artist or to the weaver of fictions.  Imagination is thus the abstract action, that is, the most intense action, of the intellect.

When I run over in my mind, and in the order of their service, the first seven presidents of the United States, Washington, Adams, Jefferson, Madison, Monroe, Adams, Jackson, I exert only memory.  The moment I begin to compare or contrast one with another, or to give the character of any of them, I put into play the higher, the imaginative action; for, to draw an historical character, the facts collected by memory must be shaped and colored and organized, the details gathered must be combined into a whole by the intellect, which being a mere tool, the success of the result (the tool being of a temper to do the work laid on it) will depend on the quality of the powers that handle it, that is, on the writer’s gifts of sympathy.