How does Descartes use the Watermark argument to prove the existence of the external world?
Answer by Billy Wheeler
Descartes’ “watermark”, also known as his “trademark” argument, is connected to knowledge of the external world through a number of steps in Descartes’ Meditations.
Firstly, Descartes starts with the assumption that we all have an idea of a perfect being. Such a perfect being, Descartes believes, has the traditional theistic properties of being all-powerful, all-knowing and all-loving. Next, it seems sensible to ask where this idea originated. Was it from perceptual experience or do we have it innately? Descartes’ here relies on a controversial metaphysical principle known as the “causal principle” which states that there must be as much perfection (or reality) in the effect as there is in the cause. On this basis, Descartes argues that his idea of God could not have originated from experience as we never do experience anything as perfect as God. In other words, we have no perceptual experience of a being that is all-powerful, all-knowing and all-loving. In fact, if you follow the logic of the causal principle, only an all-powerful, all-knowing and all-loving being could have caused this idea. Therefore, as we have this idea, it must have been caused by God. And so God exists.
This then connects back to knowledge of the external world because if God exists and he created us, then his all-loving nature means that he would not have created a being with faculties that routinely deceived them. When our faculties do go wrong, this is typically due to a fault of our own, such as being drunk or sick or not taking into account other physical factors. For example, we know that a stick does not really bend when submerged in water, even if it appears this way to our senses, and we can correct our beliefs accordingly. As a result, Descartes’ claims we have no reason to doubt the testimony of our senses systematically any more. And given that our senses routinely point to the existence of an external world, so we are justified in believing in its existence.