Subjective experience and knowledge is all that we truly have with certainty.
As Rene Descartes famously stated:
Cogito Ergo Sum — I think therefore I am.
The only thing I know for certain is that I’m here thinking. I can reasonably question just about everything else, including wondering if I might actually just be a brain in a vat being exposed to a virtual reality by some kind of evil scientist. But the one thing I cannot question is the primary fact that I am experiencing what I subjectively experience.
A further possible implication of the Cartesian Cogito is dualism: the idea that subjective experience is somehow an entirely different substance than the rest of the physical universe. Descartes apparently was a dualist, and modern-day thinking about consciousness still includes some dualists such as Chalmers (1995) who argues that the qualia aspect of consciousness is a hard problem that is beyond the scope of our objective knowledge: no amount of objective description about the mechanisms involved in driving conscious experiences in the brain will ever explain what it feels like (i.e., the qualia).
I actually agree with Chalmers in this respect, but critically identify the dualism not with some kind of mysterious break between the laws of physics and consciousness, but rather with the perspective dualism: qualia is just another name for the subjective nature of our own experience, and this subjective experience will forever remain beyond the scope of objective knowledge because each individual can never directly share their subjective experience with another individual. I have unique access to my own first person subjective experience of what it feels like to be my brain, whereas I only have an external, third person perspective on whatever might be happening with another individual.
In other words, subjective experience is literally what it feels like to actually be inside my very own individual brain. And no other individual could ever be inside my own brain, because I’m already here, and they’re already inside their own brain. And if they are not inside their own brain, they aren’t capable of understanding anything like subjective experience in the first place. That is not to say that all manner of physical systems don’t have any kind of subjective perspective: indeed every random blob of matter is unique at the bare level of physical reality, with its own spatiotemporal timeline. But as far as we know, some as-yet-not-quite-fully-specified properties of the human brain are required to fully understand and communicate an understanding of something like the nature of subjective experience.
See JilkOReillyIP for a fuller explication of this perspective dualism argument, which builds on earlier discussion by Nagel (1974) and others.
Despite the primacy of subjective experience and knowledge, it is still possible to develop an effective body of objective knowledge as discussed on that page.