Lectures de Potentia Restitutiva, or of Spring

A body is something that perfectly fills a specific quantity of space or extension, so as to necessarily exclude all other bodies from being contained within those same dimensions.

I therefore define a sensible body original: "sensible Body"; meaning a body that can be perceived by the senses to be a specific space or extension defended from being penetrated by another body by a power from within.

To make this more intelligible, imagine a very thin plate of iron (or something similar) one foot square. Imagine it moving with a vibratory motion original: "Vibrative motion" forward and backward across its flat side for the length of one foot. If it moves with such a swift motion that it does not permit any other body to enter the space within which it vibrates, this will create such an essence as I call, in my sense, a cubic foot of "sensible body." This differs from the common notion of a body in the same way that a cubic foot of space defended by this vibrating plate differs from a cubic foot of iron that is solid throughout. I suppose, therefore, that the particles that compose all bodies owe the greatest part of their sensible or potential extension to a vibratory motion.

I do not suppose this vibratory motion to be inherent or inseparable from the particles of a body, but rather communicated by impulses given from other bodies in the universe. I only suppose this: that the magnitude or bulk of a body makes it receptive to one specific motion that is communicated, and not to any other. That is, every particle of matter, according to its specific or present magnitude, is receptive to this or that particular motion and no other; thus, magnitude and the receptivity of motion seem to be the same thing.

To explain this by a comparison or example: suppose a number of musical strings, such as A, B, C, D, E, etc., are tuned to certain tones, and a like number of other strings, such as a, b, c, d, e, etc., are tuned to those same sounds respectively. String A shall be receptive to the motion of string a, but not to that of b, c, or d. In the same way, B shall be receptive to the motion of b, but not to the motion of...