1. H. Enderton, A Mathematical Introduction to Logic, Academic Press, New York-London, 1972.
2. A. Margaris, First Order Mathematical Logic, Blaisdell, Waltham, Massachusetts, 1967.
3. E. Mendelson, Introduction to Mathematical Logic, van Nostrand, Princeton, New Jersey, 1964.
4. L. Wittgenstein, Philosophical Investigations, Basil Blackwell, Oxford, 1968.