with the apprehension of spectres in the dark, may, perhaps, be said to reason, and to reason naturally too: But then it must be in the same sense, that a malady is said to be natural; as arising from natural causes, tho' it be contrary to health, the most agreeable and most natural situation of man. The opinions of the antient philosophers, their fictions of substance and accident, and their reasonings concerning substantial forms and occult qualities, are like the spectres in the dark, and are deriv'd from principles, which, however common, are neither universal nor unavoidable in human nature. The modern philosophy pretends to be entirely free from this defect, and to arise only from the solid, permanent, and consistent principles of the imagination. Upon what grounds this pretension is founded must now be the subject of our enquiry.
The fundamental principle of that philosophy is the opinion concerning colours, sounds, tastes, smells, heat and cold; which it asserts to be nothing but impressions in the mind, deriv'd from the operation of external objects, and without any resemblance to the qualities of the objects. Upon examination, I find only one of the reasons commonly produc'd for this opinion to be satisfactory, viz. that deriv'd from the variations of those impressions, even while the external object, to all appearance, continues the same. These variations depend upon several circumstances. Upon the different situations of our health: A man in a malady feels a disagreeable taste in meats, which before pleas'd him the most. Upon the different complexions and constitutions of men: That seems bitter to one, which is sweet to another. Upon the difference of their external situation and position: Colours reflected from the clouds change according to the distance of the clouds, and according to the angle they make with the eye and luminous body. Fire also communicates the sensation of pleasure at one distance, and that of pain at another. Instances of this kind are very numerous and frequent.
