Francis A. Schaeffer on Absolutes
The God Who Is There, (Downers Grove, IL: InterVarsity Press, 1968), p26.
What were these presuppositions? The basic one was that there really are such things as absolutes. [The last generation] accepted the possibility of an absolute in the area of Being (or knowledge), and in the area of morals. Therefore, because they accepted the possibility of absolutes, though people might have disagreed as to what these were, nevertheless they could reason together on the classical basis of antithesis. They took it for granted that if anything was true, the opposite was false. In morality, if one thing was right, its opposite was wrong. This little formula, "A is A" and "if you have A, it is not non-A," is the first move in classical logic. If you understand the extent to which this no longer holds sway, you will understand our present situation.