either to the right or left, forwards or backwards, then in no point of view can they be at rest.

That is the correct mode of describing them, he replied.

Then none of these objections will confuse us, or incline us to believe that the same thing at the same time, in the same part or in relation to the same thing, can act or be acted upon in contrary ways.

Certainly not, according to my way of thinking.

Yet, I said, that we may not be compelled to examine all such objections, and prove at length that they are untrue, let us assume their absurdity, and go forward on the understanding that hereafter, if this assumption turn out to be untrue, all the consequences which follow shall be withdrawn.