Philosopher Michael Dummett believed that knowledge or truth is a human-made concept. Said another way, philosophical realism claims that something can be true even if there are no people around to experience it, and Dummett rejects this view in favor of an "anti-realism". Why does Dummett believe this?
Realism
For Dummett, to say something is "real," "really there," "objectively there," or "there no matter what humans have to say about it" is to say the following: there are truths that are outside our verifiability. This is to say that to say "X is really true," the truth-ness of X doesn't depend on our being able to realize if it is true or not. This has intuitive appeal because there are disagreements about the world and what is "really there." The realist would say that if your statements do not line up with reality, then your statements aren't true.
So far, so good.
The Problem
But actually, not so good. For Dummett, the problem is that we can only ever talk about (or think about!) things that we can verify. Said another way, we can only talk about what we can talk about! If reality means that there are things out there which are true unrelated to our abilities to verify, talk about, or perceive them, then we can't verify, talk about, or perceive them at all! As such, one loses nothing by saying that a reality outside our statements or perceptions doesn't exist.
Implications
The implications of anti-realism sound, actually, quite similar to contextualism. That is, the truth of a statement is not its correspondence with reality, but is instead tied to the particular method of verification. For example, the truth of the statement "It is raining" is verified by standing outside and feeling water droplets fall on you. What this can't do is tell you that it is raining "objectively speaking." All it can do is say "'It is raining' is true if you verify it with sound, smell, sight, feeling, etc." If I experience things associated with "raining," then I can confidently state that I am justified in believing it, but nothing further than that.