"Nos autem non artibus sed philosophæ consulentes, deque potentiis non manualibus sed naturalibus scribentes," &c.
And again, nearly to the same effect in the Scholium, which follows the laws of motion, "Cæterum mechanicam tractare non est hujus instituti"
In the third law of motion he has on the contrary been supposed to speak of this force from an ambiguity in the signification of the words actio and reactio. By these, however, Newton certainly meant a mere vis motrix or pressure, as he himself explains them. "Quicquid premit vel trahit alterum, tantundem ab eo premitur vel trahitur. Si quis lapidem digito premit, premitur et hujus digitus a lapide," &c. The same meaning is equally evident from his demonstration of the third corollary to the laws, in which he asserts that the quantitas motûs of two or more bodies estimated in any given direction is not altered by their action upon each other. The demonstration begins thus:
"Etenim actio eique contraria reactio æquales sunt per legem tertiam, ideoque per legem secundam æquales in motibus efficient mutationes versus contrarias partes." Now, if he had considered the third law as implying equality of more than mere moving forces, there could have been no occasion to refer to the second law, with a view thence to deduce the equality of momenta produced.
Some authors however have interpreted the third law differently, and accordingly have expressed a difficulty in comprehending the simple illustration given by Newton. When they say that action is equal to reaction, they mean not only that the instantaneous intensity of the moving forces, or pressures opposed to each other, are necessarily equal, but
