In theory, everything should work perfectly.

In theory it is possible, but in practice it is very difficult.

There is demonstrable evidence to support the theory.

Darwinism is a theory of biological evolution.

Jurisprudence is the study and theory of law.

The idea underlies his theory.

The preponderance of evidence supports the theory.

The scientist provided a disproof of the theory.

She is studying psychological theories.

It is important to combine theory with practice.