The Doctor himself says in his preface to the disquisitions on matter and spirit, first published in 1777, that though he had entertained occasional doubts on the intimate union of two substances, so entirely heterogeneous as the Soul and the Body, the objections to the common hypothesis, did not impressively occur to him, until the publication of his treatise against the Scotch Doctors, which was in 1774. Those doubts indeed could hardly avoid occurring to any person who had carefully perused Hartley’s Essay on Man, first published in 1749, and Dr. Law’s appendix before mentioned in 1755. Dr. Hartley has shewn with a weight of fact and argument amounting to demonstration, that all the phenomena of mind, may be accounted for from the known properties and laws of animal organization; and notwithstanding, that for some reason or other he has so far accommodated his work to vulgar prejudice, as to adopt the theory of a separate Soul, though in a very objectionable form, it is evidently a clog upon his system, and unnecessary to any part of his reasoning. Substitute PERCEPTION, and his theory is compleat. Nor indeed is it possible to reject this. Constant concomitance is the sole foundation on which we build out; inference of necessary connection: we have _no_ evidence of the latter, but the former. Perception manifestly arises from, and accompanies animal organization; the facts are of perpetual occurrence, and the proof from induction is compleat. Hartley having laid a sufficient foundation to conclude (as Dr. Priestley has done) that the natural appearances of the human system might be fully explained by means of Perception and Association, without the redundant introduction of the common hypothesis, Dr. Law a few years afterward compleatly proved to the christian world that though Life and Immortality were brought to light by the christian dispensation, the common theory of a separate immaterial and immortal soul, was not necessary to, or countenanced by the christian doctrine. Dr. Law seems by his preface, to have been fearful of the consequences of expressing the whole of his opinion on this abstruse subject, and confines himself in his appendix to the examination of the passages of Scripture usually referred to in favour of the Soul’s immortality. This appendix I believe was first added to the _third_ edition of his Considerations on the Theory of Religion, published in 1755. Against Dr. Priestley, any ground of popular obloquy would be eagerly laid hold of by the Bigots of the day. The doubts expressed in the examination of Drs. Reid, Oswald, and Beattie, excited so much obloquy, as to render it necessary for Dr. Priestley to review his opinions, and renounce or defend them. The result was, the disquisition on matter and spirit, the first volume containing a discussion of the question of materialism, the second that of liberty and necessity.