An Enquiry Concerning Human Understanding (1748), David Hume argues that inductive logic is unjustified. Inductive logic is the process of making observations and then drawing larger conclusions from limited data
Critique of Pure Reason (1781), Immanuel Kant provides another challenge: It is impossible for humans to distinguish between reality (what he called the "noumenon") and our perception of reality (what he called the "phenomenon").
When astrophysicists make a claim like, "All stars are flaming balls of hydrogen and helium," that grand, sweeping claim is based on observing lots and lots of stars and observing the same thing over and over again. But they haven't observed all the stars in the universe. Further, there is no guarantee that future stars will resemble past stars. So how can they really know for sure?