Thomas Reid on the Mind as That Which ThinksEssays on the Intellectual Powers of Man, by Thomas Reid (Phillips, Sampson, and Company, 1855, orig. 1785), p.5.
By the mind of a man we understand that in him which thinks, remembers, reasons, wills. The essence both of body and of mind is unknown to us. We know certain properties of the first, and certain operations of the last, and by these only we can define or describe them. We define body to be that which is extended, solid, movable, divisible. In like manner we define mind to be that which thinks. We are conscious that we think, and that we have a variety of thoughts of different kinds; such as seeing, hearing, remembering, deliberating, resolving, loving, hating, and many other kinds of thought, all which we are taught by nature to attribute to one internal principle; and this principle of thought we call the mind or soul of a man.