On the Subject Matter of Philosophy

Most disciplines have a very specific, known (and in some cases quite narrow) subject matter. The subject matter of the discipline specifies a domain over which research might take place. For biology, for example, the Greek origins of the term suggest the domain of study is life. Psychology is the study of the mind.

While it's not always the case we can define clear-cut boundaries, there are definite subject matters which we can point and say "that's a question that the psychologists should investigate".

This raises the question of what counts as the genuine subject matter of philosophy. In a broad sense, the term philosophy means "love of wisdom". It seems to encompass all that might be learned or discovered. For example, in its early stages, physics was often referred to as "natural philosophy".

While I am very much sympathetic to this idea, in practice, philosophers have tried to separate their discipline from other disciplines. So what is it that makes philosophy a unique discipline?