Let me guess... fan of Descartes, right?
Look, it's a lovely argument to have - that we as human beings can obtain no real truths, because all truth must by definition be subjective, and therefore flawed, because we are inherently flawed, and prone to the vagaries of our own psychologies and individual bias. They did that well in The Matrix.
It's also pretty much a load of old hogwash.
There are certain universal, unambiguous truths that exist, which are extremely 'solid'. The aforementioned law of gravity, for example.
I guarantee you, I can line up a thousand second year philosophy students on a bridge - who all know what an ontological crisis is as well as they know their own mother's names - and not one of them will jump off that bridge to prove how nothing in this universe is really true.
How do I know we all see the colour blue the same? Because light particles interact with the atomic structure of the object they strike, and the blue particles in the spectrum wavelength are reflected, while the others pass through.
How do I know Batman V Superman is very probably a mediocre movie, that does disservice to both its source material and its fans? Because I have seen it to make up my own mind, have seen the consensus from 300 independent reviews, and have seen the collapse of the BO.
To quote someone a lot smarter than any of us:
Quod Erat Demonstradum.