Philosophers disagree a lot in their specific definitions, but most definitions assume it is the study of the broadest and most general questions, like what is truth, what is reality, what is the good, and so on.

Also, any particular field you study has philosophical assumptions, like about the reliability of sense data. The reason modern science is so fabulously successful is the philosophers who established it got rid of the supernaturalist assumptions of previous science, and added the assumption that reality is mathematically describable.

What Smolin and Unger are saying is that the particular philosophical assumptions behind string theory and a lot of related physics are mistaken. This would seem to be at least possible.

> Philosophy is the practice of looking into questions without clear-cut answers.

Indeed, or the practice of asking questions about the nature of questions, ie. what questions are meaningful, well-formed, yield knowledge, etc. in any given subject.

When you understand the nature of the questions in a particular subject, then you're no longer practicing philosophy, you're gathering knowledge of that subject, ie. math, physics, chemistry, etc.