The second famous argument from doubt involved the certainty of our "phenomenological" perceptions. these are perceptions that do not come from one of our five senses being stimulated by something outside of our minds. this argument from doubt is called

The answer to the blank space is the dream argument.

The concept that the question is referring to is the Cartesian doubt of Rene Descartes. It refers to skepticism of one's beliefs and it is one of the main basis for Descartes' famous cogito ergo sum argument - I think, therefore I am.

The dream argument mentioned here is established as a way for Descartes to prove that our senses that we trust to distinguish reality and illusion cannot be fully trusted, since the act of dreaming is a phenomenon that occurs as a by product of the senses.