Samkhya is usually counted as a dualistic philosophy. When we think of dualism in the West we think of René Descartes, who was a substance dualist. Descartes held that there are two kinds of things in the world: Mind and Matter. It's tempting to try to appropriate Asian philosophical notions to Western categories, but caution is warranted. For one thing, substance dualism seems to encounter a serious problem. For it seems as though our minds and bodies interact in various ways, e.g. with physical events (like hitting one's thumb with a hammer) causing mental events (like pain). But it's hard to see how a physical event can have any effect on the mind unless the mind is also a physical thing.
Prakriti, on the other hand, consists of all the possible objects of consciousness: Rocks, trees, penguins, #2 pencils, and so on. But according to Samkhya, the mind is also among the objects of consciousness. In addition to being conscious of the external world, I am also conscious of my own mind and its contents. Of course, this is not a novel claim. What is novel is that Samkhya ends up with a different division than the one we find in Descartes. It posits no distinction between mind and body; instead it distinguishes between consciousness and the body-mind. Thus Samkhya appears to be in rough agreement with the materialist tradition in Western philosophy by placing mind and body in the same category.
Samkhya takes the mind to have the ability to discriminate environmental phenomena (e.g. telling the difference between red and green light), focus attention, and control bodily movements- all of the functions normally associated with what has been called the “easy problem of consciousness.” However, according to Samkhya, the mind is not actually conscious. The body-mind, without Purusha, is what some Western philosophers have referred to as a philosophical zombie: It would be capable of performing all of the usual functions of a human being, without their being accompanied by any conscious experience. Conscious experience is made possible by Purusha.
(My reference to zombies may cause some of my readers to compare Samkhya's dualism to property dualism. Property dualism does not suppose that mind and body are separate substances; it insists instead on a distinction between mental and physical properties. There is much to be said about this comparison, but I cannot explore it here.)
Think of Prakriti, the world of experience and particularly the mind, as being like a machine that is functioning in a dark room. Now imagine a light drawing near to the machine. This light represents Purusha, the Self- it is consciousness, and it illuminates the machine of the mind. Shining in the light of consciousness, the mind appears to be conscious. It thinks, “I am the light.” But this is a mistake. At best, the mind only participates in consciousness, giving it concrete expression. Hence a Sanskrit term for mind, “citta,” which as I understand it - I am no Sanskrit scholar - refers to reified consciousness, or consciousness made concrete, as opposed to the “pure” or “root” consciousness (cit) of Purusha.
All of this is interesting theory, but problems lurk, particularly if we suppose that Samkhya's dualism is a form of substance dualism. There does not seem to be any problem here with mind-body interaction, since mind and body fall under the same category in Samkhya. But the interaction problem seems to emerge at a different level- as a problem with the interaction of consciousness and the body-mind. The analogy I have used of the light shining on the machine- which is rooted in an analogy made in the classic Yoga literature- suggests that we should understand the conscious light of Purusha as interacting causally with an otherwise-unconscious Prakriti. It seems to me that this is not possible if Purusha and Prakriti turn out to be different substances.
However, it seems to me that Samkhya need not embrace substance dualism. The distinction it makes between Purusha and Prakriti is a practical one, and the practice in which it is grounded is the practice of yoga. Samkhya, like much of Indian philosophy, is concerned to give an analysis of the human condition and in particular, of human suffering and the means to remedy it. (Its account competes with the one given by Buddhism, which insists on the nonexistence of any transcendental self.)
The cause of suffering, according to Samkhya, is the association of Purusha, the conscious Self, with the body-mind. Though we are the subjects of experience, we mistakenly identify with the objects of our experience- with our mental life, with our bodies, and to some extent with the people and things we take to be ours. We are conscious beings who are, in a sense, trying to be something that is unconscious. We are trying to be zombies, and this is painful. The dualism of Samkhya is committed to nothing more than the possibility of psychologically disassociating ourselves from mental and physical objects. This disassociation begins when we notice that there is, at least, a conceptual distinction that can be made between ourselves and the objects of our experience, and it finds its fruition in yoga practice.
The Yoga Sutras of Patanjali, tr. Vivekananda