[…] the universal and necessary rules of thought in general can concern merely its form and not in any way its matter. Accordingly, the science that contains these universal and necessary rules is merely a science of the form of our cognition through the understanding, or of thought. Kant, Jasche Logic (Logic: A Manual for Lectures), p.12

Logic inquires into the form of thought, as separable from and independent of the matter of thought of. De Morgan, On the Syllogism (1858)

What is of concern to logic is not the special content of any particular relation, but only the logical form. Frege

Thus the absence of all mention of particular things or properties in logic or pure mathematics is a necessary result of the fact that this study is, as we say, “purely formal”. Russell, Introduction to Mathematical Philosophy, p.198

The progress achieved by axiomatics consists in its having neatly separated the logical-formal form from its objective or intuitive content; according to axiomatics the logical-formal alone forms the subject-matter of mathematics, which is not concerned with the intuitive or other content associated with the logical-formal. Einstein, Geometrie and Erfahrung, p.28

[…] since we are concerned with the concept of the logical, i.e., formal, consequence, and thus with a relation which is to be uniquely determined by the form of the sentences between which it holds, this relation cannot be influenced in any way by empirical knowledge, and in particular by knowledge of the objects to which the sentence X or the sentences of the class K refer. The consequence relation cannot be affected by replacing the designations of the objects referred to in these sentences by the designations of any other objects. Tarski, On the Concept of Logical Consequence, p.414-415

These passages exemplify a tradition, going back at least to Kant and encompassing philosophers and logicians with wildly different philosophical views, of demarcating logic by its formality. I call this tradition logical hylomorphism. Logical hylomorphism is a tradition or current, not a thesis, because its advocates understand the formality of logic in very different ways. MacFarlane, What does it mean to say that Logic is Formal?, p.21