Philosophy vs belief?


I just read that Solipsism is a syndrome as well as a philosophy.

Exactly what is the difference between believing in something (as in the above syndrome), and having it be your philosophy?

I’ve always just assumed philosophy was synonymous with beliefs.

the rational investigation of the truths and principles of being, knowledge, or conduct.

Whereas solipsism seems to be the belief in something without external input.
the external world and other minds cannot be known and might not exist outside the mind