One of the main objects of the logicians' inquiries has traditionally been various provinces of what might be called logical truth. The Stoics started investigations into another province of logical truth, which had not been systematically studied by Aristotle. Logic of Relations was hardly studied at all before de Morgan. All the constants of the Logic of Propositions occur among the constants of the Logic of Properties, and all the constants of the Logic of Properties among the constants of the Logic of Relations. A substantial contribution to the clarification of the idea of logical truth was made by Wittgenstein in his Tractatus logico- philosophicus. Logical truth could therefore be said to consist in agreement with every one of a number of mutually exclusive and collectively exhaustive possibilities. The concept of a tautology works very well for the purpose of clarifying the idea of logical truth or 'independence of content' in propositional logic.