[5] Keith Campbell said that Armstrong's contributions to metaphysics and epistemology "helped to shape philosophy's agenda and terms of debate", and that Armstrong's work "always concerned to elaborate and defend a philosophy which is ontically economical, synoptic, and compatibly continuous with established results in the natural sciences".

[10] Armstrong married Jennifer Mary de Bohun Clark in 1982 and had step children.

[7][12] In 1950, Armstrong formed an Anti-Conscription Committee with David Stove and Eric Dowling (R. E. Dowling), all three former students of John Anderson, the Australian philosopher, and all later to be academic philosophers, who then began to support conscription and also believed that anti-conscription opinions ought to be suppressed.

From this fundamental assumption flows a rejection of abstract objects including Platonic forms.

[17] Armstrong's development as a philosopher was influenced heavily by John Anderson, David Lewis, and J. J. C. Smart,[18] as well as by Ullin Place, Herbert Feigl, Gilbert Ryle and G. E.

[19] Armstrong collaborated with C. B. Martin on a collection of critical essays on John Locke and George Berkeley.

[23] Armstrong's universals are "sparse": not every predicate will have an accompanying property, but only those which are deemed basic by scientific investigation.

Armstrong then suggests that a supervenience relation exists between these second order properties and the ontologically authentic universals given to us by physics.

How Armstrong's theory of universals deals with relations with varying adicities has been raised as an issue by Fraser MacBride.

[28] This primitive results in a vicious regress for both kinds of nominalisms,[29] Armstrong suggests, thus motivating his states-of-affairs based system that unites properties by postulating a primitive tie of instantiation [30] based on a fact-ontology, called states of affairs.

He also says that "Plato in his later works, Aristotle and the Scholastic Realists were ahead of contemporary philosophy in this matter, although handicapped by the relative backwardness of the science and the scientific methodology of their day".

[35] Armstrong argues that states of affairs are distinct things in ontology because they are more than the sum of their parts.

Without states of affairs instantiating the particulars and universals (including relations), we cannot account for the truth of the one case and the falsity of the other.

Under the theory of Armstrong, Tooley and Dretske, there is a relation of necessity between the universals ravenhood and blackness, rather than there being a relationship with every single raven.

He initially was attracted to Gilbert Ryle's The Concept of Mind and the rejection of Cartesian dualism.

'[48] Stephen Mumford said that Armstrong's A Materialist Theory of the Mind "represents an authoritative statement of Australian materialism and was, and still is, a seminal piece of philosophy".

[51] The intuitions that lead to this kind of externalism led Alvin Plantinga towards an account of knowledge that added the requirement for 'properly-functioning' cognitive systems operating according to a design plan.

She both believes and disbelieves her husband is dead: it just happens that one of her two beliefs is justified, true and satisfies some knowledge conditions.

[53][54] Armstrong presents a response to Colin Radford's modified version of the "unconfident examinee" example.