Non-contradiction is an universal logical rule #sorrynotsorry

I take this to be a fundamental logical principle of rationality: that whatever you are already committed to believing restricts what else you can believe (without revising what you are already committed to believing). This is enough to derive some version of the principle of non-contradiction, all that is required is recognizing that this relation be recognized as being none other but what we call “negation”.

The basis motivation behind the main and only premise of this argument is that rationality has to do not only with what one ought to believe or not ex nihilo, but what one ought to believe or not given what we already do believe or not. This basic principle can be decomposed in four different principles:

  1. What one believes restricts what one can believe or not
  2. What one believes restricts what one ought to believe or not
  3. What one does not believe restricts what one can believe or not
  4. What one does not believe restricts what one ought to believe or not
Let us focus on the first one of these. In semi-formal terms, it says there is at least one set of beliefs A for which there is at least another set of beliefs B* such that if one believes A then one can believe also B*. Now, let B be the complement of B. This means that if one believes A then one cannot also believe B. In other words, A and B are incompatible as beliefs. There is an incompatibility relation between A and B. Since incompatibility is (at least one kind of) negation, whenever we have this sort of relation, we can call B the negation of A. Thus, what this principle says is that if one believes A then one cannot believe also its negation, which is exactly what the principle of non-contradiction says. Thus, to reject it is to reject the very general principle that what one believes restricts what else one can believe, i.e., to reject the basic idea that the rationality of belief cannot be evaluated but relative to other beliefs.

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