A **scalene** triangle abc might also be employed, or a tetrahedron.

32 was first proved in a general way by the Pythagoreans; but, on the other hand, we learn from Geminus that the ancient geometers observed the equality to two right angles in each kind of triangle - in the equilateral first, then in the isosceles, and lastly in the **scalene** (Apoll.

A formula such as the equality of the interior angles of a triangle to two right angles is only scientifically known when it is not of isosceles or **scalene** triangle that it is known, nor even of all the several types of triangle collectively, but as a predicate of triangle recognized as the widest class-concept of which it is true, the first stage in the progressive differentiation of figure at which it can be asserted.° Three points obviously need development, the nature of definition, its connexion with the syllogism in which the middle term is cause or ground, and the way in which we have assurance of our principles.

25) that he perfected the things relating to the **scalene** triangle and the theory of lines.