I.- I. The Fundamental Criterion for the Soundness of Arguments.- 1. Introduction.- 2. Fundamental criterion for the soundness of arguments.- 3. The formal character of logic.- 4. The transition to mathematical logic.- 5. Construction of a fragment of modern logic.- 6. Natural deduction.- 7. Supplementary considerations.- II. Inferential and Classical Logic.- 8. Semantic and operational aspects of meaning.- 9. Purely implicational logic.- 10. Deduction problems and deductive tableaux.- 11. Truth-value problems and semantic tableaux.- 12. Inferential and classical logic - Peirce's Law.- 13. Other aspects of meaning.- 14. Informal logic : G. Mannoury, A. Naess, Ch. Perelman.- III. Proof by Contradiction.- 15. Introductory remarks.- 16. Conversion of closed tableaux into natural deductions.- 17. The negation.- 18. Semantic and deductive tableaux.- 19. Final considerations - completeness of classical purely implicational logic.- IV. The Problem of Locke-Berkeley.- 20. An example.- 21. Statement of the problem and attempts at its solution.- 22. The exposition method of Aristotle.- 23. Appeal to the method of semantic tableaux.- 24. The completeness problem - informal and heuristic deduction methods.- V. On the So-Called 'Thought Machine'.- 25. Introduction.- 26. Binary arithmetic and logic.- 27. Specific operations of the human intellect.- 28. Prehistory.- 29. Difficulties.- 30. Development of modern formal logic.- 31. Automatization of reasoning.- 32. Analysis of the difficulties.- 33. An example.- 34. Heuristics or methodology.- 35. Methodology.- 36. Concluding remarks.- II.- VI. The Paradoxes.- VII. Reason and Intuition.- VIII. Formalized Language and Common Usage.- IX. Considerations about Logical Thought.- X. Constants of Mathematical Thought.- Sources.- Index of Names.- Index of Subjects.