HOW DO WE KNOW THAT “REALITY” is not something we are just imagining but not real?
Until modern times that may have been a debated philosophical question but no one, except those considered mentally unbalanced, lived as if reality was something in our heads. That’s because in pre-modern times meaning and significance were understood to be in things themselves apart from what our minds perceived or attributed to them. But in modern times there has been a shift in the location of meaning. Significance is now often thought to be the property of the mind which perceives meaning or even imposes meaning on things. And so we hear people say, “That’s your truth but it isn’t mine.”
This is the kind of thinking that makes the movie, The Matrix, possible. Wikipedia describes The Matrix as a “science fiction action film… (that) depicts a dystopian future in which reality as perceived by most humans is actually a simulated reality called "the Matrix", created by sentient machines to subdue the human population, while their bodies' heat and electrical activity are used as an energy source” (As of May 30, 2018, Wikipedia).
The idea that meaning is a property of the mind has now moved into mainstream thinking. For example, we are asked to accept that a person who was born a male (genetically) is actually a female if that is what s/he believes to be true. The deciding factor is not DNA but what that person thinks. And we are asked not only to allow someone to live in the reality they perceive, we also must live in that reality with them. To do otherwise is unloving and intolerant, maybe even phobic.
So, is reality located in the mind or in things themselves? The modern mindset can only stand if one rejects the idea that there is an intelligent Designer who created everything, determines the meaning and significance of everything, and that he’s the boss of everything. You must say, rather, that authority to decide meaning has moved from God to, well, me. You cannot have it both ways. One of those views is the truth and one is a delusion.