The 6 Meditations
Meditation 1
Doubt everything. We must assume everything is a deception and cannot trust that anything is true. We must build a foundation on top of this by starting with what we can concretely 100% say we know is true.
Meditation 2
Cogito. Basically (but not quite) “I think, therefore I am”. He argues that the one indubitable truth is that we exist, because we are thinking and out thoughts exist.
The self as a thinking thing is the most certain thing there is.
Wax argument
What you perceive in physical objects is not grasped by the senses or imagination, but by the intellect alone. If the mind is what knows physical objects, then the mind is more directly known than physical objects. The mind is what you know first and most clearly, and physical objects are known only indirectly through it.
Meditation 3
Causal Adequacy Principle: There must be at least as much reality in the cause as in the effect.
Applied to ideas, the cause of an idea must have at least as much formal reality as the idea has objective reality.
A thing as it actually exists = formal reality.
A thing as it is represented in an idea = objective reality.
Formal reality is reality a thing has by existing. Objective reality refers to the reality of what the idea represents, its content.
The idea of God represents an infinite, perfect substance. That idea has maximal objective reality. So its cause must be something with maximal formal reality, meaning an actually infinite, perfect being. Humans, being a finite and imperfect substance, lack enough reality to be the cause of such an idea. Therefore the idea must have been placed in people by something that actually is infinite and perfect, namely God.
Meditation 4
Meditation 5
Meditation 6
Argues for Dualism - a separation of mind and body as completely different and independent substances. The body is divisible while the mind is indivisible.