LXVII. We shall conclude this discourse, by pointing out three capital errors, which are derived from want of proper attention in making experiments. The first is, that of taking for the effect, what in reality is the cause, and taking for the cause, what is nothing more than the effect. The second consists in taking for the cause, something that comes in by accident, and which has no influence whatever. The third consists, in between two effects proceeding from one and the same cause, taking one of them for the cause of the other. I shall give examples of these three errors, in observations appertaining to medicine, which are said to be derived from experience, because mistakes in this branch, are generally attended with worse consequences, than those in other ordinary physical matters. LXVIII. It happens, that a man feels an ardent and extraordinary thirst upon himself, without being able to assign any apparent cause of it; he drinks water to great excess, and in a few hours is seized with a fever, or an acrid fluxion. It is common in such cases, to attribute the indisposition to the excessive drinking of water, and to apprehend, that was the cause of the disease; but this was so far from being the case, that the indisposition was rather the cause of his drinking to that excess. But I would have it observed, that I speak of an instance, in which the thirst was not brought on by any manifest or apparent cause, such as the having used any violent exercise, or having been exposed to some great heat, either of the sun or fire, or having been a long time without drinking, or having eaten something very salt. I say that agreeable to this state of the case, it is very clear, that the thirst must have arisen from some internal cause; but the question is, from what cause? Why it could be from no other, than a morbific disposition, that had begun to prevail within his body; or let us express it in another manner, and say, that it proceeded from an acrid or salt humour, which had just begun to get in motion, and by vellicating the fibres that produce the sensation of thirst, had by that means excited it. Every preternatural and extraordinary effect, demands a preternatural and extraordinary cause to occasion it; and if we suppose that the thirst was such an effect, and we cannot assign any external cause to which we can attribute it; we must conclude that it proceeded from some preternatural internal cause; which in all probability, must be the morbific disposition.