Direct proof: derive the target from the accepted premises without assuming its contradictory.
Proof by reductio: assume the contradictory of the target, derive an impossibility, and reject the assumption.
For example, the claim that every square has four sides can be grounded directly in the definition of a square. A reductio instead assumes a square lacking four sides and derives a conflict with that definition.
This is an additional instructional example. The text itself uses “Every human is an animal” to illustrate the technique.