I remember reading somewhere that philosophical thinking was about being able to simultaneously consider one opinion and its opposite, or something of the like. Where was that? I am pretty sure it was in a philosophy book. The idea has grown on me, and now I would like to come back to the book from which it came.
This sounds like the Hegel's view of speculative thought. According to Hegel, it is a natural movement of thought to turn to its opposite ("negation"), and for the two to coexist until the opposition is resolved in negation of negation, "sublation":"Hegel calls on speculative thought: two contradictory elements are held together, uplifted and sublated without completely destroying one another. Speculative thought seeks to avoid the idealism inherent in reflective thought and allows one to think in concrete terms about how things work, both in the present, real world and in history".
This dialectical method is at the heart of Hegel's philosophy, which asserts as a “necessary function of reason”, “the necessity of the contradiction which belongs to the nature of thought determinations”. Moreover, we should abandon “tenderness for the things of this world”, and the idea that “the stain of contradiction ought not to be in the essence of what is in the world”.
The modern term for asserting two opposing opinions simultaneously is dialetheism, however that is stronger than merely "considering" them.
To my best knowledge, a sentiment somewhat akin to this statement could be found in JS Mill's On Liberty, a utilitarian argument on why society should not silence the expression of contrarian opinions, partly to defend it from individual fallibility:
First: the opinion which it is attempted to suppress by authority may possibly be true. Those who desire to suppress it, of course deny its truth; but they are not infallible. They have no authority to decide the question for all mankind, and exclude every other person from the means of judging. To refuse a hearing to an opinion, because they are sure that it is false, is to assume that their certainty is the same thing as absolute certainty...
...Unfortunately for the good sense of mankind, the fact of their fallibility is far from carrying the weight in their practical judgment, which is always allowed to it in theory; for while every one well knows himself to be fallible, few think it necessary to take any precautions against their own fallibility, or admit the supposition that any opinion, of which they feel very certain, may be one of the examples of the error to which they acknowledge themselves to be liable. (On Liberty, II.3-4)
Two philosophies come to mind.
- Pyrrhonism (see Sextus Empiricus) uses opposing arguments to leave one in a state of suspension of belief, and hence achieve ataraxia.
- Hegelian dialectic believed that concepts contained their contradictions and chose to reconcile the two by finding a higher concept that contained both concept and contradiction.