In his Fifth Meditation, Descartes makes a solid argument to disprove the idea that truth is found only through our senses. To use one of this examples, we do not know triangles are real because we have seen them. We can imagine a perfect triangle, but no such triangle occurs...
In his Fifth Meditation, Descartes makes a solid argument to disprove the idea that truth is found only through our senses. To use one of this examples, we do not know triangles are real because we have seen them. We can imagine a perfect triangle, but no such triangle occurs naturally such that it could be observed and known through our senses. Similarly, we can imagine shapes and other things that we have never seen or experienced, but to say they are not "something" would be misguided. We have imagined them, so they must be something even if we cannot observe them.
Descartes does not argue that the triangle is an object created willingly by his imagination. It is just something that is there, and there are known facts regarding it that exist no matter how you imagine the triangle (e.g. the angles necessarily add up to 180 degrees). However, this is learned and understood, he argues, because it was already "inside" of you. You can imagine a triangle, and things we now know to be true about triangles would still be true (even in your initial imagination) before it is consciously learned or known. Therefore, what is true can exist irrespective of sensory reality.
You can imagine shapes that are not perceived by your senses, and things will be ultimately true about them. This truth is not dependent on its physical existence (in other words, it is not dependent on you perceiving its existence with your senses). The truth is dependent on the image or idea conjured by your imagination. You did not consciously or willingly create the object, nor did you create the math that is necessary for it to exist. So it must be "something," even though you cannot perceive it with your senses. Since you can only with your intellect or mind, but there are still truths about it, it must exist even though it is not possible to experience it through your senses. Such a mode of knowing is typically classified as a priori.
He draws from this discussion that this is why he knows there is a God. We imagine a perfect God, and though we cannot see Him with our senses, the idea of God conjures an idea of perfect existence. "Existence" is necessary—part of God's essence, like 180 degrees are part of the essence of a triangle—so therefore, God must exist. There is no idea of God without perfection and existence, so those ideas must be "something" and therefore must exist even though God cannot be perceived with our senses. In other words, we can conjure the idea of something that is absolutely perfect in all ways, so it must be something and therefore exist. This is what we think of when we think of a sole God.
So, Descartes guarantees that everything that is true must be something.