Carl Schmitt, Glossarium: Aufzeichnungen der Jahre 1947-1951, hrsg. v. Medem (Berlin: Duncker und Humblot, 1991), S. 213 (14. I. 49):
Das Feindschaftpotential des Denkens ist unendlich. Denn man kann nicht anders als in Gegensätzen denken. Le combat spirituel est plus brutal que la bataille des hommes.
The enmity potential of thought is infinite. For one cannot think otherwise than in oppositions. Spiritual combat is more brutal than a battle of men. (tr. WFV)
There is something to this, of course. Philosophy in particular sometimes bears the aspect of a blood sport. But thinking is just as much about the reconciliation of oppositions as it is about their sharpening. A good thinker is rigorous, precise, clear, disciplined. These are virtues martial and manly. But there are also the womanly virtues, in particular, those of the midwife. Socratic maieutic is as important as ramming a precisely formulated thesis down someone's throat or impaling him on the horns of a dilemma. The Cusanean coincidentia oppositorum belongs as much to thought as the oppositio oppositorum.
There is more to philosophy than "A thing is what it is and not some other thing." There is also, "The way up and the way down are the same."
But it is no surprise to find the unrepentant Nazi onesided on the question. We shall have to enter more deeply into the strange world of Carl Schmitt.