What do theoretical and practical reason have in common, and where do they differ?

Both solve questions by reflecting on reasons — i.e. by arguing; they differ only in the kind of question: truth of a belief vs. the rightness of an action.

The two reasons are siblings, not opposites:

• Common ground: both involve the capacity to resolve questions by reflecting on reasons — and that, concretely, is arguing. Whether you're deciding what's true or what to do, you weigh grounds.
• The difference is the target question:
  • Theoretical reason → the truth of a belief/conviction.
  • Practical reason → the normative/moral rightness of an action.

Tip: As critical thinkers we're told to master both — because people constantly face two interlocking questions: what to think, and what to do. Theoretical reflection changes your beliefs (via logic and argument); practical reason yields well-grounded actions (via normative ethics).