A treatise of human nature: being an attempt to introduce the experimental method of reasoning into moral subjects. ... [pt.3]
Hume, David, 1711-1776.

A note to Vol. I. page 118. line 8. to these words (impressions and ideas.)

As long as we confine our speculations to the appearances of objects to our senses, with∣out entering into disquisitions concerning their real nature and operations, we are safe from all difficulties, and can never be embarrass'd by any question. Thus, if it be ask'd, if the invisible and intangible distance, interpos'd betwixt two objects, be something or no∣thing: 'Tis easy to answer, that it is some∣thing, viz. a property of the objects, which affect the senses after such a particular man∣ner. If it be ask'd, whether two objects, having such a distance betwixt them, touch or not: It may be answer'd, that this de∣pends upon the definition of the word, touch. If objects be said to touch, when there is nothing sensible interpos'd betwixt them, these objects touch: If objects be said to touch, when their images strike contiguous parts of the eye, and when the hand feels both objects successively, without any inter∣pos'd motion, these objects do not touch. The appearances of objects to our senses are Page  310 all consistent; and no difficulties can ever arise, but from the obscurity of the terms we make use of.

IF we carry our enquiry beyond the ap∣pearances of objects to the senses, I am afraid, that most of our conclusions will be full of scepticism and uncertainty. Thus if it be ask'd, whether or not the invisible and intangible distance be always full of body, or of something that by an improvement of our organs might become visible or tangible, I must acknowledge, that I find no very de∣cisive arguments on either side; tho' I am inclin'd to the contrary opinion, as being more suitable to vulgar and popular notions. If the Newtonian philosophy be rightly un∣derstood, it will be found to mean no more. A vacuum is asserted: That is, bodies are said to be plac'd after such a manner, as to receive bodies betwixt them, without impul∣sion or penetration. The real nature of this position of bodies is unknown. We are only acquainted with its effects on the senses, and its power of receiving body. Nothing is more suitable to that philosophy, than a modest scepticism to a certain degree, and a fair confession of ignorance in subjects, that exceed all human capacity.