In First Meditation: On what can be called into doubt., Descartes concluded that the assumption where his former opinions are “utterly false and imaginary” was necessary to avoid being deceived by what he had learned before.
The belief that we, human beings, exist is mostly based on our senses. The world we believe we live in is made up of things we see, things we hear, and things we feel every day. But what if these senses are merely created by the imagination of our brains. How could one distinguish if he is in a dream or reality? Descartes argued that although there wasn’t a reliable way, the imagination must come from real things. He continued, “studies of the simplest and most general things” such as arithmetic and geometry were relatively certain and indubitable, whereas “sciences dealing with things that have complex structures” were doubtful, including physics, astronomy, medicine, etc. During the development of the knowledge of humankind, a deeper understanding of the universe came from the questions and doubts about the old beliefs, it is reasonable to raise educated doubt about any of the former beliefs.