This is the mode by which, without recurring to grace, to inspirations, to visions, to supernatural notions, we can render ourselves an account of that uncertain, that wavering state into which we sometimes behold persons fall, when there is a question respecting their superstition, who are otherwise extremely enlightened. Frequently, in despite of all reasoning, momentary dispositions re-conduct them to the prejudices of their infancy, upon which on other occasions they appear to be entirely undeceived. These changes are very apparent, especially under infirmities, in sickness, or at approach of death. The barometer of the understanding is then frequently obliged to fall. Those chimeras which he despised, or which in a state of health, he set down at their true value, are then realized. He trembles, because his machine is enfeebled; he is irrational because his brain is incapable of fulfilling its functions with exactitude. It is evident these are the actual causes of those changes which the priests well know how to make use of against what they call incredulity; from which they draw proofs of the reality of their sublimated opinions. Those conversions, or those alterations, which take place, in the ideas of man, have always their origin in some derangement of his machine; brought on either by chagrin or by some other natural or known cause.
Submitted to the continual influence of physical causes, our systems invariably follow the variations of the body; we reason well when the body is healthy--when it is soundly constituted; we reason badly when the corporeal faculties are deranged; from thence our ideas become disconnected, we are no longer equal to the task of associating them with precision; we are incapable of finding principles, or to draw from them just inferences; the brain, in fact, is shaken; we no longer contemplate any thing under its actual point of view. It is a man of this kind, who does not see things in frosty weather, under the same traits as when the season is cloudy, or when it is rainy; he does not view them in the same manner in sorrow as in gaiety; when in company as when alone. Good sense suggests to us, that it is when the body is sound, when the mind is undisturbed by any mist, that we can reason with accuracy; this state can furnish us with a general standard, calculated to regulate our judgment; even to rectify our ideas, when unexpected causes shall make them waver.