The modest enquirer therefore, without pretending to put himself on an equality with these illustrious men, may be forgiven, when he permits himself to suggest a few doubts, and presumes to examine the grounds upon which he is called upon to believe all that is contained in the above passages. Now the foundations upon which astronomy, as here delivered, is built, are, first, the evidence of our senses, secondly, the calculations of the mathematician, and, in the third place, moral considerations. These have been denominated respectively, practical astronomy, scientific, and theoretical. As to the first of these, it is impossible for us on this occasion not to recollect what has so often occurred as to have grown into an every-day observation, of the fallibility of our senses. It may be doubted however whether this is a just statement. We are not deceived by our senses, but deceived in the inference we make from our sensations. Our sensations respecting what we call the external world, are chiefly those of length, breadth and solidity, hardness and softness, heat and cold, colour, smell, sound and taste. The inference which the generality of mankind make in relation to these sensations is, that there is something out of ourselves corresponding to the impressions we receive; in other words, that the causes of our sensations are like to the sensations themselves. But this is, strictly speaking, an inference; and, if the cause of a sensation is not like the sensation, it cannot precisely be affirmed that our senses deceive us. We know what passes in the theatre of the mind; but we cannot be said absolutely to know any thing, more. Modern philosophy has taught us, in certain cases, to controvert the position, that the causes of our sensations are like to the sensations themselves. Locke in particular has called the attention of the reasoning part of mankind to the consideration, that heat and cold, sweet and bitter, and odour offensive or otherwise, are perceptions, which imply a percipient being, and cannot exist in inanimate substances. We might with equal propriety ascribe pain to the whip that beats us, or pleasure to the slight alternation of contact in the person or thing that tickles us, as suppose that heat and cold, or taste, or smell are any thing but sensations. The same philosophers who have called our attention to these remarks, have proceeded to shew that the causes of our sensations of sound and colour have no precise correspondence, do not tally with the sensations we receive. Sound is the result of a percussion of the air. Colour is produced by the reflection of the rays of light; so that the same object, placed in a position, different as to the spectator, but in itself remaining unaltered, will produce in him a sensation of different colours, or shades of colour, now blue, now green, now brown, now black, and so on. This is the doctrine of Newton, as well as of Locke.