Was MLK a philosopher? History doesn't really consider him one, but he did have a lot of views regarding fairness and justice, and his ideas were very influential upon the development of civil rights and equality.

It's an interesting question, but especially at the meta-level. I've been thinking about how it should be answered and here's my tentative theory.

One way someone can count as a philosopher is if people who count uncontroversially as philosophers by and large count the person as a philosopher. In this case: if philosophers generally counted MLK as a philosopher, that would be enough to settle the question. As it happens, this isn't the case for MLK (at least, not that I'm aware.)

Another way is if the person's work is the kind of work that philosophers would generally count as philosophy. That's a bit vague, but here's a sort of operationalized version. Suppose we took samples of the person's work and presented them to lots of philosophers (ideally without telling them whose work it was.) If philosophers tended to agree that the work (however valuable it may be) isn't philosophy, that would make a good case for saying no; the person isn't a philosopher. If philosophers tended to agree that the work is philosophy, that would make a good case for saying that the person is a philosopher. What would happen if we tried this test in MLK's case is not something I have an opinion on; I'm simply not familiar enough with his writing.

Now of course, there are things we'd expect to find in work that philosophers counted as philosophy: careful argumentation, attention to concepts, concern with questions that philosophers tend to discuss, discussed in the ways and at the level that philosophers tend to discuss them... But making a reliable list and deciding how the different items ranked would be controversial. My suggestion is a way of cutting the Gordian knot. The simple version is: if philosophers would judge that the work is philosophy, the person who did the work is a philosopher. That's a rough answer, but since it seems plausible to me, it must contain some wisdom!