How if we go on to argue: therefore, _Every man is apt to blunder, prone to confusion of thought, inured to self-contradiction_?
Practically, the substitution of identities must be left to candour and good-sense; and may they increase among us.

Formal Logic is, no doubt, safest with symbols; should, perhaps, content itself with A and B; or, at least, hardly venture beyond Y and Z.
Sec.5.The principle of Contradiction is usually written symbolically, thus: _A is not not-A_.

But, since this formula seems to be adapted to a single term, whereas we want one that is applicable to propositions, it may be better to write it thus: _B is not both A and not-A_.

That is to say: _if any term may be affirmed of a subject, the contradictory term may, in the same relation, be denied of it_.

A leaf that is green on one side of it may be not-green on the other; but it is not both green and not-green on the same surface, at the same time, and in the same light.
If a stick is straight, it is false that it is at the same time not-straight: having granted that two angles are equal, we must deny that they are unequal.
But is it necessarily false that the stick is 'crooked'; must we deny that either angle is 'greater or less' than the other?
How far is it permissible to substitute any other term for the formal contradictory?
Clearly, the principle of Contradiction takes for granted the principle of Identity, and is subject to the same difficulties in its practical application.