Hyl. No, certainly.
Phil. How then can sound, being a sensation, exist in the air, if by the air you mean a senseless substance existing without the mind?
Hyl. You must distinguish, Philonous, between sound as it is perceived by us, and as it is in itself; or (which is the same thing) between the sound we immediately perceive, and that which exists without us. The former, indeed, is a particular kind of sensation, but the latter is merely a vibrative or undulatory motion in the air.
Phil. I thought I had already obviated that distinction, by the answer I gave when you were applying it in a like case before. But, to say no more of that, are you sure then that sound is really nothing but motion?
Hyl. I am.
Phil. Whatever therefore agrees to real sound, may with truth be attributed to motion?
Hyl. It may.
Phil. It is then good sense to speak of motion as of a thing that is loud, sweet, acute, or grave.
Hyl. I see you are resolved not to understand me. Is it not evident those accidents or modes belong only to sensible sound, or sound in the common acceptation of the word, but not to sound in the real and philosophic sense; which, as I just now told you, is nothing but a certain motion of the air?
Phil. It seems then there are two sorts of sound—the one vulgar, or that which is heard, the other philosophical and real?