Metaphysics describes two questions:

What is there?  &   What is it like?


So for example this is answered easily. We know say that bananas exist and we know that they are yellow, edible, and have skin.

We have attributed and collected several things that we know for sure about bananas.
However! how can we be sure about the existence of bananas? Or the existence of anything in that matter? Let us break this down, most of what we understand about say bananas is through our senses. We understand the color of bananas because we see yellow, we taste the sweetness of bananas. Descartes argues that maybe our senses are not the most reliable and he argues for three levels of doubt.

The first is the doubt of illusion People have definently been tricked by their senses before, as say, a colorblind person can get a color wrong, or the example Descartes uses is a stick in water that looks bent, but is straight. However, this theory is easily disproved because we can use our other senses to bypass illusion most of the time.



The second level of doubt is dreams. Do you know when you are dreaming? Do you know if you are dreaming right now? Our consciousness literally plays with our senses during dreams, so that it is hard to tell during whether or not we are dreaming. However, Desecartes argues that because dreams are not anachronistic (not chronilogical), that it is easy to distinguish dreams as not reality.



The third level, is what he calls the "Malignant Demon", but in modern day terms sounds like simulation theory. The previous two theories suggest that there is a way for our senses to be tricked. In his case, Desecartes argue that a demon may be controlling his experience of the world, that there may not be a world at all even. It is very possible in fact, there we could currently be in a simulation, everything we know currently has the possibility to be a lie.
The one truth we do know is that we are thinking, that we hold the ability to have these conversations. So the one truth we do know is quite aptly