Hyl. But, all this while you have not considered that the quantity of Matter has been demonstrated to be proportioned to the gravity of bodies[883]. And what can withstand demonstration?
Phil. Let me see how you demonstrate that point.
Hyl. I lay it down for a principle, that the moments or quantities of motion in bodies are in a direct compounded reason of the velocities and quantities of Matter contained in them. Hence, where the velocities are equal, it follows the moments are directly as the quantity of Matter in each. But it is found by experience that all bodies (bating the small inequalities, arising from the resistance of the air) descend with an equal velocity; the motion therefore of descending bodies, and consequently their gravity, which [pg 460] is the cause or principle of that motion, is proportional to the quantity of Matter; which was to be demonstrated.
Phil. You lay it down as a self-evident principle that the quantity of motion in any body is proportional to the velocity and Matter taken together; and this is made use of to prove a proposition from whence the existence of Matter is inferred. Pray is not this arguing in a circle?
Hyl. In the premise I only mean that the motion is proportional to the velocity, jointly with the extension and solidity.
Phil. But, allowing this to be true, yet it will not thence follow that gravity is proportional to Matter, in your philosophic sense of the word; except you take it for granted that unknown substratum, or whatever else you call it, is proportional to those sensible qualities; which to suppose is plainly begging the question. That there is magnitude and solidity, or resistance, perceived by sense, I readily grant; as likewise, that gravity may be proportional to those qualities I will not dispute. But that either these qualities as perceived by us, or the powers producing them, do exist in a material substratum; this is what I deny, and you indeed affirm, but, notwithstanding your demonstration, have not yet proved.
Hyl. I shall insist no longer on that point. Do you think, however, you shall persuade me the natural philosophers have been dreaming all this while? Pray what becomes of all their hypotheses and explications of the phenomena, which suppose the existence of Matter[884]?
Phil. What mean you, Hylas, by the phenomena?
Hyl. I mean the appearances which I perceive by my senses.
Phil. And the appearances perceived by sense, are they not ideas?