1. The Turn to the Science
The problem with substance dualism is that, given what we know about how the world works, it is hard to take it seriously as a scientific hypothesis. We know that in humans consciousness cannot exist at all without certain sorts of physical processes going on in the brain. We might, in principle, be able to produce consciousness in some other physical substance, but right now we have no way of knowing how to do this. And the idea that consciousness might be produced apart from any physical substrate whatever, though conceivable, just seems out of the question as a scientific hypothesis.

2. How We Can Save Mind as a Substance
It is not easy to make the idea that the mind is a separate substance consistent with the rest of what we know about the world. Here are three ways of trying to do it, each with a different conception of the mind.

3. How We Can Save Mind as a Substance
First, divine intervention. Physical science is incomplete. Our souls are something in addition to the rest of the world. They are created by divine intervention and are not part of the physical world as described by science.

4. How We Can Save Mind as a Substance
Second, quantum mechanics. The traditional mind-body problem arises only because of an obsolete Newtonian conception of the physical. On one interpretation of quantum measurement, consciousness is required to complete the collapse of the wave function and thus create quantum particles and events. So some form of consciousness is not created by the rest of nature, rather it is essential for the creation of nature in the first place. It is a primitive part of nature required to explain brain processes and everything else.

5. How We Can Save Mind as a Substance
Third, idealism. The universe is entirely mental. What we think of as the physical world is just one of the forms that the underlying mental reality takes.

6. Idealism
In the philosophy of perception, idealism is contrasted with realism in which the external world is said to have a so-called absolute existence prior to, and independent of, knowledge and consciousness. Epistemological idealists (such as Kant), it is claimed, might insist that the only things which can be directly known for certain are just ideas.

7. Idealism
In the philosophy of mind, idealism is contrasted with materialism in which the ultimate nature of reality is based on physical substances. Idealism and materialism are both theories of monism as opposed to dualism and pluralism.

8. Ancient Philosophy
Plato: Forms and forms
Modern Philosophy
Descartes (epistemological idealism)

9. Occasionalism
Malebranche disagreed that if the only things that we know for certain are the ideas within our mind, then the existence of the external world would be dubious and known only indirectly. He declared instead that the real external world is actually God. All activity only appears to occur in the external world. In actuality, it is the activity of God. For Malebranche, we directly know internally the ideas in our mind. Externally, we directly know God's operations. This kind of idealism led to the pantheism of Spinoza.

10. Pre-Established Harmony
Leibniz: Monads are indecomposable, individual, subject to their own laws, un-interacting, and each reflecting the entire universe. Monads are centers of force; substance is force, while space, matter, and motion are phenomenal. For Leibniz, there is an exact pre-established harmony or parallel between the world in the minds of the alert monads and the external world of objects. God, who is the central monad, established this harmony and the resulting world is an idea of the monads’ perception.

11. George Berkeley’s Idealism
Bishop Berkeley, in seeking to find out what we could know with certainty, decided that our knowledge must be based on our perceptions. This led him to conclude that there was indeed no "real" knowable object behind one's perception, that what was "real" was the perception itself. This is characterised by Berkeley's slogan: "Esse est aut percipi aut percipere" or "To be is to be perceived or to perceive", meaning that something only exists, in the particular way that it is seen to exist, when it is being perceived (seen, felt, etc.) by an observing subject.

12. Immanuel Kant’s Idealism
Immanuel Kant held that the mind shapes the world as we perceive it to take the form of space-and-time. It is said that Kant focused on the idea drawn from British empiricism (and its philosophers such as Locke, Berkeley, and Hume) that all we can know is the mental impressions, or phenomena, that an outside world, which may or may not exist independently, creates in our minds; our minds can never perceive that outside world directly. Kant made the distinction between things as they appear to an observer and things in themselves, "... that is, things considered without regard to whether and how they may be given to us ... ."

13. Immanuel Kant’s Idealism
... if I remove the thinking subject, the whole material world must at once vanish because it is nothing but a phenomenal appearance in the sensibility of ourselves as a subject, and a manner or species of representation.
– Critique of Pure Reason A383

14. Immanuel Kant’s Idealism
Kant's postscript to this added that the mind is not a blank slate, tabula rasa, (contra John Locke), but rather comes equipped with categories for organising our sense impressions. Perhaps this Kantian sort of idealism opens up a world of abstractions (i.e., the universal categories minds use to understand phenomena) to be explored by reason, but perhaps, in sharp contrast to Plato's, confirms uncertainties about a (un)knowable world outside our own minds. We cannot approach the noumenon, the "Thing in Itself" (German: Ding an Sich) outside our own mental world. (Kant's idealism is called transcendental idealism.)