by pointing out a variety of ocular deceptions, in which, although we know that we are imposed upon by the appearances of things, we may nevertheless be said to believe for the moment that the things are as they appear. But he has merely illustrated his opinion, he has not attempted to vindicate or establish it upon rational grounds, or to explain it by means of any law of our intelligence. These grounds and this law I shall now endeavour to lay before you; for Mr Stewart's opinion, singular and somewhat paradoxical though it be, appears to me to be founded in truth, and to be susceptible of a satisfactory explanation. I think that Dr Reid's opinion may also be justified; in short, that the doctrines of the two philosophers on this point may be reconciled with one another by means of the principle which I am about to point out to you.
2. In proceeding to point out to you the grounds on which I think the soundness of this opinion may be upheld, I commence by remarking that there is a particular circumstance connected with the exercise of Perception and of Imagination to which your attention must be directed. This circumstance I would call the law of contrast between perception and imagination, and between the objects of perception and the objects of imagination. This law may be either present or absent when these faculties are at work, When this law is present, and when the imagination is at work, then I hold with Dr Reid
