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(44.) It has been proved, that inability to change the quantity of motion is a consequence of inertia. The inability to change the direction of motion is another consequence of this quality. The same cause which increases or diminishes motion, would also give motion to a body at rest; and therefore we infer that the same inability which prevents a body from moving itself, will also prevent it from increasing or diminishing any motion which it has received. In the same manner we can show, that any cause which changes the direction of motion would also give motion to a body at rest; and therefore if a body change the direction of its own motion, the same body might move itself from a state of rest; and therefore the power of changing the direction of any motion which it may have received is inconsistent with the quality of inertia.

(45.) If a body, moving from A, fig.3. to B, receive at Ba blow in the direction CBE, it will immediately change its direction to that of another line BD. The cause which produces this change of direction would have put the body in motion in the direction BE, had it been quiescent at B when it sustained the blow.