Theory in medicine is principally despised by two sets of practitioners, the ignorant and the lazy; the last are unwilling, and the first are unable, to acquire the knowledge of it; and, amidst all their boasted contempt, it will be found, that they principally differ from others in implicitly adopting whatever opinions they first receive. Theory is natural to the human mind; and those who are accustomed to think cannot be satisfied without assigning a reason for every phænomenon which falls under their observation. Theory by no means precludes observations: On the contrary, it leads to a more minute attention to facts; and in this way opinions, of themselves ill founded, have often given rise to useful discoveries in practice. For these reasons, in treating of mercury in the venereal disease, I have esteemed it necessary to begin with some observations on its operations. There are few medicines, with regard to the operation of which all practitioners are agreed. Mercury, as well as others, has, in this respect, been a subject of dispute. It is indeed the misfortune of medicine, that many points of great consequence still remain in a very doubtful state. But it affords at least some chance of arriving at truth, that, in the present age, it is never esteemed criminal to differ even from the highest authorities: On the contrary, those whose authority should deservedly have the greatest weight, are unwilling that any farther deference should be paid to their opinion than arises from conviction. He therefore who imagines, that an ill-grounded opinion is generally received, has nothing to restrain him from a candid inquiry. But arguments adduced in favour of any opinion will have little weight, if another be previously received as true. To ensure therefore a fair examination, it is, in the first place, requisite to remove prepossession. These remarks will sufficiently account for the attention I have here bestowed in attempting to refute an opinion different from that which I endeavour to establish. It is an opinion taught, and at this place very generally received, with regard to the operation of mercury, that the good effects it produces, in the cure of _lues venerea_, are to be ascribed to the evacuation which it occasions. After attentively examining this hypothesis, I cannot help thinking that it is ill founded. Notwithstanding, therefore, the authority by which it is supported, I have, with freedom, stated what to me seem strong objections against it. After having endeavoured to overturn one hypothesis, I have attempted to establish another. The opinion I support is by no means new. The honour of invention therefore must be totally resigned to others; but the praise of broaching novelties is always to be relinquished when it comes in competition with the support of truth; and I should consider myself as having done a great deal, were I able to give satisfactory evidence of any important but doubtful point.