I have heard that evolution is both a fact and a theory.  Is common descent considered the "fact" of evolution and natural selection the "theory"?

If common descent is considered the "fact" of evolution, how is that possible without direct observation?  Isn't most of the evidence inferential?

The compliation here is the use of 'theory' and 'fact' in different contexts.

In common day-to-day language we use 'theory' generally to mean a hunch - an unsupported idea, something that had yet to be tested, or one with only tenuous support "I have a theroy that if we buy player X for our team, we will be much better next season". You know the kind of thing, even scientists are happy to use the term in that sense.

However, in science, a theory is something very different. A theory is a complex set of ideas that have undergone rigorous testing, analysis, assessment, retesting, data collection etc. etc. and survived. A scientific theory can explain all the data that it is supposed to (evolution says nothing about gravity for example, a gravitational anomoly would cause problems from physicists, not biologists) and can predict future discoveries. It is not a guess, a hunch, an opinion, or even something that has been supported by a little data but not much study has been done (thius would be a hypothesis). We have the theory of gravity, the theory of evolution, the theory of relativity, atomic theory etc.

Evolution is also a day-to-day fact. It happens. Just as gravity is a fact. Things falls, planets rotate, light bends etc. When someone asks you when a ball thrown up falls to Earth, you would not say it was the theory of gravity. Just gravity. You would not consider it *not* to be a fact.

In short, evolution is a scientific theory, and a fact of reality. Just as gravity is a 'theory' (how and why objects are attaracted to one another) and a fact (they *are* attracted to one another, and as predicted by the theory). Evolutionary theory is how we explain what is going on, but it *is* happening. I hope this cleans it up. Really, it is just a problem of semantics, and more importantly the misues or misunderstanding of the words by some people in different contexts.

All that said, it is also true that while commonality of descent is firmly established by multiple lines of evidence, the mechanism or mechanisms by which it happens are less fully understood, and always open to refinement.