To illustrate this distinction by example, let us suppose ourselves, in walking across a lawn, to turn our eyes to a particular _point_, and to perceive there _an oak_. That is to say, the presence of the oak, or rather of the light reflected from it, occasions a certain new state of the mind, which we call a sensation of _vision_, an affection, which belongs to the mind alone, indeed, but of which we have every reason to suppose, that the mind, _of itself_, without the presence of light, would not have been the subject. The peculiar sensation, therefore, is the result of the presence of the light reflected from the oak; and we perceive it, because the mind is capable of being affected by external things. But this affection of the mind, which has an external object for its immediate cause, is not the only mental change which takes place. Other changes succeed it, without any other external impression. We compare the oak with some other tree which we have seen before, and we are struck with its superior magnificence and beauty;--we imagine how some scene more familiar to us would appear, if it were adorned with this tree, and how the scene before us would appear, if it were stripped of it;--we think of the number of years, which must have passed, since the _oak_ was an _acorn_;--and we moralize, perhaps, on the changes, which have taken place, in the little history of ourselves and our friends, and, still more, on the revolutions of kingdoms,--and the birth and decay of a whole generation of mankind,--while it has been silently and regularly advancing to maturity, through the sunshine and the storm. Of all the variety of states of the mind, which these processes of thought involve, the only one, which can be ascribed to an external object as its direct cause, is the primary perception of the oak; the rest have been the result not immediately of any thing external, but of preceding states of the mind;--that particular mental state, which constituted the perception of the oak, being followed immediately by that different state, which constituted the remembrance of some tree observed before, and this by that different state which constituted the comparison of the two; and so successively, through all the different processes of thought enumerated. The mind, indeed, could not without the presence of the oak,--that is to say, without the presence of the light which the oak reflects,--have existed in the state which constituted the perception of the oak. But as little could any external object, without this primary mental affection, have produced immediately, any of those other states of the mind, which followed the perception. There is, thus, one obvious distinction of the mental phenomena; as in relation to their causes, external or internal; and, whatever other terms of subdivision it may be necessary to employ, we have, at least, _one_ boundary, and know what it is we mean, when we speak of the external and internal affections of the mind.