A Treatise of Human Nature - Volume 2

The classification is the same as that which *I formerly used when I distinguished them into impressions of sensation and reflection. Original impressions, or impressions of sensation, are those which arise in the soul without any prior perception; they come from the constitution of the body, from the animal spirits In 18th-century physiology, "animal spirits" were thought to be fluids in the nerves that transmitted sensory information and movement., or from the application of objects to the external organs. Secondary, or reflective impressions, are those which proceed from some of these original ones, either immediately or by the introduction of an idea. Of the first kind are all the impressions of the senses, and all bodily pains and pleasures. Of the second kind are the passions and other emotions resembling them.

It is certain that the mind, in its perceptions, must begin somewhere; and since impressions precede their corresponding ideas, there must be some impressions which appear in the soul without any introduction. As these depend upon natural and physical causes, examining them would lead me too far from my present subject and into the sciences of anatomy and natural philosophy original: "natural philosophy"; this term refers to what we now call the natural sciences, particularly physics and biology.. For this reason, I shall confine myself here to those other impressions which I have called—

* Book I. Part I. Section 2.