A dualist is one who believes that the facts which
he considers—whether they be the facts of the world
in general or a particular class of them—cannot be
explained except by supposing ultimately the existence
of two different and irreducible principles. For exam-
ple, dualists in anthropology explain facts about man
by two fundamental causes: reason and the passions,
soul and body, or freedom and determinism; in the
theory of knowledge, dualists explain knowledge by
the meeting of two different realities: subject and ob-
ject; in religious cosmology, they picture the world as
dominated by the perpetual conflict of a good and an
evil power, both of which have always existed.
There are various kinds of dualism, depending on
the different subjects of reflection or research. How-
ever, the subjects in which the term dualism is most often
employed are the history of religions and philoso-
phy. Thomas Hyde seems to have invented the term
“dualist,” which he uses in his history of the religion
of the ancient Persians (Historia religionis veterum
Persarum, 1700) in order to designate the men who
think that God and the devil are two coeternal princi-
ples. The term was later used in this same sense by
Pierre Bayle and by Leibniz. Christian Wolff first ap-
plied it to the philosophers who considered the body
and the soul as two distinct substances: “The dualists
(dualistae) are those who admit the existence of both
material and immaterial substances, that is, they con-
cede the real existence of bodies outside the ideas of
the souls and defend the immateriality of these souls”
(Psychologia rationalis [1734], Sec. 39). Most philoso-
phers employ the word in Wolff's sense, whereas most
historians of religions have retained the meaning of
“dualism” which it had in Hyde.
The word “dualism” then has two principal mean-
ings: (1) religious and (2) philosophical. In sense (1)
it designates religions such as Zoroastrianism of the
later Avesta and of the Pahlavi books; in sense (2)
dualism applies to philosophies such as Cartesianism.
It must be noted that these are very different doctrines,
from which it would be possible to draw even contra-
dictory consequences. For example, the dualism of soul
and body or of mind and matter might lead to the
denial of the existence of an absolutely evil mind (the
devil) and even of an evil principle. Matter is not in
itself evil for the dualistic philosophers; and a pure
mind can hardly be evil for those who think that the
cause of error, and consequently of evil, is the mixture
of mind with matter or the inversion of their legitimate
hierarchic order.
Since the two doctrines are distinct, we must con
sider them separately. However, it may seem that they
are united in some systems, for instance in Manicheism,
in which God's adversary is often personified but is
also identified with matter. But in Manicheism there
is something else, in addition to these two forms of
dualism. Manicheism proceeds from Gnosticism, and
Gnostic dualism—although some scholars held it to be
a synthesis of Hellenic (that is to say of philosophical)
and of Zoroastrian dualism—is neither exactly a philo-
sophical dualism, nor a religious one belonging to the
Zoroastrian type, nor a synthesis of both. It is in fact
a third genus, which consists essentially in the opposi-
tion of God and the world. We shall therefore handle
it in a third section. This peculiar form of dualism may
be considered as belonging principally to the history
of Christian theology.
