state of decomposition it may have a tendency to become volatile again, or it may lie dormant entirely; and this last will explain what is quoted from Dr. Russel, p. 178, that sometimes commerce may be carried on with infected places without danger. [96] See p. 128, note. Thus we see that the dispute, originally begun about a word, involves at last a matter of the utmost importance; for, if it be found unreasonable to believe that any such thing as contagion exists or can exist, it follows of course that it is also unreasonable to take any precautions against it. Mr. McLean even goes a step beyond those who deny the existence of contagion; for we find him also denying that putrid effluvia can produce epidemics; according to which doctrine, it seems, we may not only safely visit places accounted the most dangerous on account of infection, but live in all manner of filth and nastiness with impunity. It is plain that no person can ever prove that it is impossible for contagion or any thing else to _have an existence_. Indeed if nothing had ever induced people to believe that it did exist, it would have been superfluous to say any thing about it. But when we have innumerable testimonies to the contrary; when the opinions of the greatest physicians, as Dr. Lind, Dr. Clarke, Dr. Mead, Dr. Sydenham, Dr. Fordyce, Dr. Russel, &c. agree that not only the plague, but every kind of fever, is infectious; when we know from the analogy of nature that contagion _may_ exist; when we know that there certainly _are_ powers in nature able to produce it; is all this to be thrown aside merely on the strength of a theory, and a theory too which can never be proved? for it is impossible to prove the _non-existence_ of any thing, much less the _impossibility_ of its existence. The lives of mankind are too precious to be sported with on philosophical theories; and prudence will always suggest, that wherever danger may at any time arise, there it is proper to be on our guard. Dismissing at length the subject of contagion in general, we now enter upon the question, Whether doth it appear from fair investigation of testimony, that the plague has, at any time, been communicated by contagion or not? And here I shall confine myself to what has been adduced by Dr. P. Russel on the subject; for, if we find that the disease has only _once_ been introduced by contagion, it signifies nothing though we were able to prove, which we never can do, that it had been _an hundred_ times bred in some other way. The matter is of too great importance to allow even a _chance_ of its importation by the neglect of the precautions necessary to prevent it.