The simplicity argument begins with the basic premise that thought is immaterial. Since thought is an object of consciousness, it is then concluded that the mind is immaterial.  To define “immaterial” the term unextended has been used in philosophical dialogs, meaning it contains no parts. Therefore, the mind is a simple substance – a single entity. From this initial conclusion three further observations can be made.  A simple substance is (1) indestructible, or it cannot be divided and maintain its identity, (2) unified, and (3) an identity. Dr. Mijuskovic has elaborated seven uses of this argument in his books The Achilles of Rationalist Arguments and Contingent Immaterialism: (1) the defense of personal immortality, (2) the unity of consciousness, (3) personal identity, (4) metaphysical idealism, (5) the immateriality of meanings, (6) the freedom of consciousness, and (7) internal time-consciousness. Each use corresponds roughly to historical periods from Plato through Sartre.

The earliest formulation of the argument occurs in Plato’s Phaedo 78b and following. Its designation as the “Achilles” by Kant indicates his consideration of it as the strongest among the rationalist arguments to which he wrote his Paralogisms in the Critique of Pure Reason. Kant’s efforts revealed the weakness of its use in defense of personal immortality, but the remaining six uses have been evident in philosophical discourses through the present. In addition to Plato and Kant, other notable figures Mijuskovic explores include Aristotle, Plotinus, Bayle, Leibniz, Berkeley, Hume, Schopenhauer, Brentano, James, Husserl, Bergson and Sartre.

In the past decade there has been one major development in this chronicle by a team of authors under the direction of Thomas Lennon and Robert Stainton published as The Achilles of Rationalist Psychology, which focuses upon the second use in defense of the unity of consciousness exclusively. Lennon and Stainton restructured Dr. Mijuskovic’s definition into an analytical structures based upon broader or narrower versions of the argument. The narrowest Achilles argument is as follows:

P1: Unification of representations takes place,
P2: Only a simple, unified substance can unify representations
C1: The human soul or mind is a simple unified substance.

Although this analytical structure does not address the indestructibility or identity of the mind, it is a development, nonetheless. Dr. Mijuskovic does not subscribe to this particular reworking, but it does suggest another way to see the basic structure of the argument through analytical lenses. As admirable as The Achilles of Rationalist Psychology is in reviving this topic, it, unfortunately, does not consider the greater breadth of its uses to which Dr. Mijuskovic addresses in his latest work entitled “The Argument from Simplicity: A Study in the History of an Idea and Consciousness.”

The Achilles has a rich history which has barely been tapped into by contemporary philosophers. The most recent contribution to the history is a forthcoming thesis focusing on the use of the simplicity argument in the works of Roderick M. Chisholm and Richard Swinburne respectively has been supervised by Dr. Mijuskovic. These combined efforts merely scratch the surface of this significant argument. Other contemporary candidates whose work in philosophy of mind or consciousness studies engages the simplicity argument include David Barnett, Dean Zimmerman, Charles Taliaferro and E. J. Lowe.