Note 2 — MILL'S LOGIC.[1]

Necessary truths multiply on us very fast. "We maintain," says Mr Whewell, "that this equality of mechanical action and reaction is one of the principles which do not flow from, but regulate, our experience. A mechanical pressure, not accompanied by an equal and opposite pressure, can no more be given by experience than two unequal right angles. With the supposition of such inequalities, space ceases to be space, form ceases to be form, matter ceases to be matter." And again he says, "That the parallelogram of forces is a necessary truth;" a law of motion of which we surely can conceive its opposite to be true. In some of these instances Mr Whewell appears, by a confusion of thought, to have given to the physical fact the character of necessity which resides in the mathematical formula employed for its expression. Whether a moving body would communicate motion to another body—whether it would lose its own motion by so doing—or what would be the result if a body were struck by two other bodies moving in different directions—are questions which, if they could be asked us prior to experience, we could give no answer whatever to—which we can easily conceive to admit of a quite different answer to that which experience has taught us to give.