To paraphrase, Descartes open his Meditations with the frustration, 'Every time we humans think we have come up with an idea that is fundamentally true, inevitably, every damn time, we end up learning something years or even centuries later that overturns that belief, and throws into question everything else we believed because of it.' He saw this as the shortcoming of science despite its power. He wanted to figure out thought so pure and true and unshakeable that no future discovery could possibly threaten its truth. The problem was where to start. He questioned everything single thing that he experienced: every sensation and belief. He was consummately cynical.
He realized that every taste, touch, smell, sight, or sound he had ever experienced might itself be an illusion. He couldn't trust even his own senses, experiences, memories. He thought some Great Demon might be the engineer of the entire universe-- that it was all an illusion designed to ensnare his soul. In modern times, due to the rise of computers and string theory, we have begun to wonder if our own universe is nothing more than a simulation being run by some greater Being the way we run experiments or simulations in order to better understand our own universe.
Not sure if you're a Rick and Morty fan, but one of the reasons hardcore fans like myself esteem this show as elevated art derives from how they weave these culturally defining philosophical ideas into episodes so effortlessly and seamlessly. In typical fashion, the show took this idea to its nihilistic, sardonic extreme in the episode where Rick introduces Morty to the arcade game "Roy" where you simulate an entire lifetime just to see what your score is. No, it's not even a Great Demon after your soul. That would have meaning: an epic clash of Good & Evil. No, your entire life could be a consumerist throwaway for (what many consider to be the lowest brow form of) entertainment. That's how meaningless this all might be. Freaking hilarious. Brilliant.
Back to Descartes, in doubting the very existence of his loved ones, and himself, he realized the one and only truth that he could not unravel was the fact that he was thinking in the first place. The very act of thinking confirmed his existence. "I am a thinking thing", IIRC, was his mantra.
That became his first step. That is solipsism; not the consideration that life is an illusion, but the belief that you can't be certain of anything beyond yourself by virtue of your own self-consciousness.
