I have a quantity of MS. in hand for Parts II and III, and hope to be able----should life, and health, and opportunity, be granted to me, to publish them in the course of the next few years. Their contents will be as follows:-- _PART II. ADVANCED._ Further investigations in the subjects of Part I. Propositions of other forms (such as "Not-all x are y"). Triliteral and Multiliteral Propositions (such as "All abc are de"). Hypotheticals. Dilemmas. &c. &c. _Part III. TRANSCENDENTAL._ Analysis of a Proposition into its Elements. Numerical and Geometrical Problems. The Theory of Inference. The Construction of Problems. And many other _Curiosa Logica_. pg-viii PREFACE TO THE FOURTH EDITION. The chief alterations, since the First Edition, have been made in the Chapter on 'Classification' (pp. 2, 3) and the Book on 'Propositions' (pp. 10 to 19). The chief additions have been the questions on words and phrases, added to the Examination-Papers at p. 94, and the Notes inserted at pp. 164, 194. In Book I, Chapter II, I have adopted a new definition of 'Classification', which enables me to regard the whole Universe as a 'Class,' and thus to dispense with the very awkward phrase 'a Set of Things.' In the Chapter on 'Propositions of Existence' I have adopted a new 'normal form,' in which the Class, whose existence is affirmed or denied, is regarded as the _Predicate_, instead of the _Subject_, of the Proposition, thus evading a very subtle difficulty which besets the other form. These subtle difficulties seem to lie at the root of every Tree of Knowledge, and they are _far_ more hopeless to grapple with than any that occur in its higher branches. For example, the difficulties of the Forty-Seventh Proposition of Euclid are mere child's play compared with the mental torture endured in the effort to think out the essential nature of a straight Line. And, in the present work, the difficulties of the "5 Liars" Problem, at p. 192, are "trifles, light as air," compared with the bewildering question "What is a Thing?"