15
The Predicate Calculus
15.1 Motivation
The propositional calculus has several limitations. We can’t, for example, express the fact that when we move block B, say, it is the same block that ON_B_C asserts is on block C. In the propositional calculus, atoms are strings that have no internal structure. In propositions about toy blocks, ON_A_B and ON_B_C are completely different with absolutely nothing in common. Even though I used mnemonic names for these atoms (to help us remember what I intend them to mean), I could just as well have used different proposition letters for them, say, P124 and Q23, respectively.
A more useful language would be one that could refer to objects in the world (such as blocks) as well as to propositions about the world. ...
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