Ontology is the branch of philosophy that explores the whole concept of existence. Thomas Aquinas argued that a reason as to why the ontological argument does not work is that we do not know what God is, Descartes disagreed with this.
Descartes, who was a very influential mathematician, philosopher and scientist, believed, similarly to Anselm and Plato previously, that human beings mind’s are imprinted with concepts such as equality, cause, shape and number as well as an understanding of what God is which we all share from birth. We have an intrinsic understanding that God is the supremely perfect being and is perfect in every way. By ‘perfect’, Descartes means omniscience, omnipotence ad omnibenevolence.
Descartes used an analogy of a triangle to explain his understanding of how this innate idea demonstrated the existence of God. He claimed that by comparing it to the way we ordinarily establish very basic truths in arithmetic and geometry, such as that the number two is even or that the sum of the angles of a triangle is equal to the sum of two right angles. We intuit such truths directly by inspecting our clear and distinct ideas of the number two and of a triangle. So, likewise, we are able to attain knowledge of God's existence simply by apprehending that necessary existence is included in the clear and distinct idea of a supremely perfect being.
Descartes also used an analogy of a mountain; he said that the existence of God is part of the essence of God, like a valley is part of the essence of a mountain. He recognised that this had its limitations, while we might be able to think of a mountain without thinking of a valley additionally, this does not mean the combination in our imagination actually exists in the real world. However, Descartes argued that his nature does not involve angles or valleys but perfections; therefore existence is a perfection because God has all the perfections, and existence