Syllogism
Helpful Wikipedia entry listing and discussing various syllogistic forms.
The Traditional Square of Opposition
Examines the history of the idea of the Square of Opposition from Aristotle to Peter Strawson. From the Stanford Encyclopedia of Philosophy.
SparkNotes on the Organon
Brief summary and discussion of the works (listed below) collectively described as Aristotle's Organon.
Aristotle: Prior Analytics
Trans. A J Jenkinson. From the Internet Classics Archive.
Aristotle: Posterior Analytics
Trans. GRG Mure. From the Internet Classics Archive.
Aristotle: On Interpretation
Trans. EM Edghill. From the Internet Classics Archive.
Aristotle: Topics
Trans. WA Pickard-Cambridge. From the Internet Classics Archive.
Aristotle: On Sophistical Refutations
Trans. WA Pickard-Cambridge. From the Internet Classics Archive.
Aristotle: Categories
Trans. EM Edghill. From the Internet Classics Archive.
Begriffsschrift
RB Jones' useful notes on Frege's revolutionary work, summarised here as "a formalised language of pure thought modelled upon the language of arithmetic."
Propositional Calculus
Wikipedia article explaining the propositional calculus.
Propositional Logic
Exercises by Professor Karlis Podnieks of the University of Latvia.
Predicate Calculus
Amusing and useful introduction, with exercises, by Stefan Waner and Steven R Costenoble (both of Hofstra University's mathematics department). Taken from their online Introduction to Logic.
First-order Logic
Wikipedia article explaining the first-order predicate calculus.
Categorical Syllogisms
Explanatory article from Garth Kemerling's Philosophy Pages.
Sets
Professor RHB Exell's excellent, somewhat technical exposition of sets and elements in logic; taken from his "Guide to Logic" and hosted by the Joint Graduate School of Energy and Environment.
The Logic of Classes
1990 article by Avi Sion, from The Logician website.
Relations
Professor Exell's excellent guide to the logic of relations; taken from his "Guide to Logic" and hosted by the Joint Graduate School of Energy and Environment.
Modal Logic
Article exploring the whole "family" of modal logics, as well as the deductive behaviour of expressions of necessity and possibility. From the Stanford Encyclopedia of Philosophy.
Aristotelian and Modern Logic
Essay by Katalin Havas considering the relationship between Aristotelian and modern logic, asking whether the latter can be said to have developed from the former, or whether a paradigm shift occurred.
Inductive Reasoning
Wikipedia article on inductive logic.
Four Varieties of Inductive Argument
from the University of North Carolina Greensboro's Department of Philosophy.
Category-Based Induction
Informative exposition of its use and problems by Javed Absar.
Fuzzy Logic Tutorial
The Seattle Robotics Society's explication of fuzzy logic, described in relation to robotic systems.
Vagueness: Its Nature and Logic
Exploration of vagueness and its relationship to paradox. From the University of St Andrews.
Logical Paradoxes
Discusses the nature of logical paradox and gives some famous examples (paradoxes of self-reference; the Hooded Man; etc). From the Internet Encyclopedia of Philosophy.
Logical Paradoxes
Fun introduction to logical paradox, presenting well-known (eg, The Liar, Achilles and the Tortoise) and more obscure ("The Specious Present," etc) paradoxes.
Paraconsistent Logic
Challenging the principle that anything follows from contradictory premises, and so to most standard logics. Article from the Stanford Encyclopedia of Philosophy.
Sense and Reference
Wikipedia article exploring these central concepts of meaning in modern logic.
Frege: On Sense and Reference
Quotations collected by Rick Grush of the philosophy department at the University of California at San Diego.
Descriptions
Stanford Encyclopedia of Philosophy article discussing various analyses of definite and indefinite descriptions ("the F" and "an F").
Bertrand Russell: On Denoting
Hosted by the University of Michigan's Center for the Study of Complex Systems.
The Journal of Symbolic Logic
JSTOR access point.
Logic Journal of the IGPL
Subscription-only journal of the Interest Group in Pure and Applied Logic.
Nordic Journal
of Philosophical Logic
Edited by the Department of Philosophy at the University of Oslo.
Notre Dame Journal of Formal Logic
A subscription-based journal; articles more than five years old are available here, at Project Euclid, which sells access to past editions of journals and individual articles in mathematics and statistics.