Logic is concerned with Forms

A Syllogism needs to be both Sound and Valid. Soundness is concerned with the Truth of the Propositions while Validity is concerned with the Form of the Syllogism as a whole.

In the example,

All Men are Mortal

Socrates is Man

Therefore, Socrates is Mortal.

each of the Propositions is True. The Form or the structure of the three Propositions taken together is also Valid. Validity is described as the knowledge that would be non-contradictory.

The importance of Form could be understood: All rational knowledge is either material or formal; the former considers some object, the latter is concerned with the form of the understanding and of the reason itself, and with the universal laws of thought in general without distinction of its objects. *

Thus , though the material or content in the Syllogism matters, it is the Form of the Syllogism that decides the Validity of the Syllogism.

*Fundamental Principles of the Metaphysics of Morals, Immanuel Kant