there was a debate for years around whether Project Management is an art or a science.
The truth is that neither are correct on their own – it is both.
Successful project managers (with a track record of more than one project) typically employ the science stuff (core PM concepts), sometimes religiously.
They also harbor a good deal of the “art” side as well. One example of the art would be the harder to define competencies or even what some people call personal qualities that good PMs have, for example judgement. You cannot be a successful project manager (of anything non-trivial) without very good judgement.
Something that is (sadly) not found in all of us.
For more resources, see the Library topic Project Management.