I really think that the 'muse' is something that many artists have interacted with, but don't really remember. What made you FIRST start drawing? Not just doodles, but really drawing and really loving it and thinking "hey, I can be good at this". I think that is the most basic form of a muse. After that there are many muses. What inspires each artist to make each piece in each individual way, different and unique unto themselves as an creative person, is yet another muse. If one were to really look at it, it may all be connected and ascribed to some higher power or spiritual or artistic force, or it may remain as completely separate and disconnected things, held deep within the creative process of each piece and each time. That really depends on the person and the artist.
No matter which artist you look at you will usually see some over-arching theme, if not a an entire style, representative of what first and continues to inspire that artist. Sometimes it's incredibly subtle, especially if that artists is now a professional, drawing for others what they want to see. Or sometimes it's all consuming in their work and shows like a great neon sign on a page. I would go as far as to say that those signs are like the fingerprints of the muse, a sign of the source of inspiration.
Personally I have always been inclined toward and attracted by darker imagery, but I didn't really find a muse or inspiration until I began to explore the relative balance between opposing forces, most notably light and darkness. Not just in the picture, as in the play of shadow and light, but also in symbols and even direct representations like demons and angels. That is not to say that I am a religious or spiritual person, although I do think that on some level I am (especially as an artist), it's just that is what captures my imagination and spurs me into creativity as an artist. It lead me as a child to turn my sadness into happy images, and later my happiness into playfully dark and twisted things. So I would have to say that personally my muse is the dichotomy of the world. It may sound a little pretentious, but that is just my perspective