I recently had a conversation wih a man who thought that nothing actually exists. He believed that we are only imagining everything and thus are the product of some kind of illusion.
Could you suggest what theory he was mentioning so that I can research more into it?
This isn't one unified theory, but a strain of thought that has reoccurred many times and in many different forms in the world of philosophy. Some of the most notable versions are found in Plato, who viewed the ordinary material world as a imperfect copy or reflection of a more true Reality composed of ideal elements (perhaps most memorably explained in the Allegory of the Cave), in Bishop Berkeley's claim that the entire world is contained in the mind of God, and in Descartes' Meditations, where he considers and discards the notion that the sensible world is an illusion created by a malign demon.
There are also elements of this point of view in Hindu and Buddhist philosophy, as well as in the Taoist writings of Lao Tzu and Chuang Tzu (see the latter's "butterfly dream" for an example).
More recently, variants of this idea formed the nucleus for the movies The Matrix, Inception, and The Truman Show among others.
The concept of Maya answers your question.
In Indian philosophies, Māyā is ... a spiritual concept connoting "that which exists, but is constantly changing and thus is spiritually unreal", and the "power or the principle that conceals the true character of spiritual reality". - Wikipedia - Maya (illusion)
The idea is summed up nicely in the Lankavatara Sutra.
In the following quote, the 'real world' is not discounted; just subjective reality is called illusion, because it is prone to error and manipulation. Nevertheless, how could the 'real world' be known, except to say something has furnished the material (matter and minds) for subjective, illusory perception.
Mahamati, since the ignorant and simple-minded, not knowing that the world is only something seen of the mind itself, cling to the multitudinousness of external objects, cling to the notions of being and nonbeing, oneness and otherness, bothness and not-bothness, existence and non-existence, eternity and non-eternity, and think that they have a self-nature of their own, all of which rises from the discriminations of the mind and is perpetuated by habit-energy, and from which they are given over to false imagination. It is all like a mirage in which springs of water are seen as if they were real. They are thus imagined by animals who, made thirsty by the heat of the season, run after them. Animals, not knowing that the springs are an hallucination of their own minds, do not realise that there are no such springs. In the same way, Mahamati, the ignorant and simple-minded, their minds burning with the fires of greed, anger and folly, finding delight in a world of multitudinous forms, their thoughts obsessed with ideas of birth, growth and destruction, not well understanding what is meant by existent and non-existent, and being impressed by the erroneous discriminations and speculations since beginningless time, fall into the habit of grasping this and that and thereby becoming attached to them.
It is like the city of the Gandharvas which the unwitting take to be a real city though it is not so in fact. The city appears as in a vision owing to their attachment to the memory of a city preserved in the mind as a seed; the city can thus be said to be both existent and non-existent. ...
It is like a wheel of fire made by a revolving firebrand which is no wheel but which is imagined to be one by the ignorant. Nor is it not-a-wheel because it has not been seen by some. By the same reasoning, those who are in the habit of listening to the discriminations and views of the philosophers will regard things born as non-existent and those destroyed by causation as existent. It is like a mirror reflecting colors and images as determined by conditions but without any partiality. It is like the echo of the wind that gives the sound of a human voice. It is like a mirage of moving water seen in a desert. In the same way the discriminating mind of the ignorant which has been heated by false-imaginations and speculations is stirred into mirage-like waves by the winds of birth, growth and destruction. It is like the magician Pisaca, who by means of his spells makes a wooden image or a dead body to throb with life, though it has no power of its own. In the same way the ignorant and the simpleminded, committing themselves to erroneous philosophical views become thoroughly devoted to the ideas of oneness and otherness, but their confidence is not Well grounded.
The modern strain goes along the lines of whether reality is a simulation, and there are many technologists taking up this perspective now. It was discussed here Are we living in a simulation? The evidence
Buddhism takes the perspective that the arising of things is founded in delusions, which result from cravings for things to be other than they are, which is founded in misconceiving the nature of our world. The fundamental metaphor of Buddhism, to attain awakening or enlightenment, references leaving a sleeping state.
I think this character you met must be thinking along the latter Buddhist lines, to say "nothing actually exists" although it should be noted, he must have a fairly tortuous definition of thing and nothing to use that precise phrasing. In Buddhism they would term that nihilism, the opposite extrem view to eternalism, both of which are avoided by Buddhism's 'Middle Way' which views things as contingently arising but lack any permanent nature. Do suggest to him he may have misunderstood the genre of thought on this. We can still make some sense even of a dream, writing off any attempt to do so can only be founded in psycology that rejects all thought and reasoning, for an instinctive decision.
It could be called a Cartesian theory. Cartesianism is the name given to the philosophical doctrine (or school) of René Descartes. Cartesians view the mind as being wholly separate from the corporeal body [source Wikipedia].
