The above quote “Reality is that which, when you stop believing in it, doesn't go away.” comes from a Philip K Dick essay; but PKD himself was not happy about that definition - and it is frequently refuted so in his own fiction and the Exegesis.
It is being refuted on a daily basis in 2020.
To define reality in terms of 'experience' is a pragmatic definition; the idea that reality will bite whoever denies it, and reward whoever honours it.
But this, in the end (if you pursue it), this principle can be seen to reduce to "reality is whatever is expedient". What 'works' cannot atheoretically be distinguished from 'what is expedient'. Always a causal path must be inferred.
And Expediency is, substantially, a matter of power.
Furthermore, the assumption is that when reality bites, we will know it is reality. But this is obviously not the case in a world where government, education, major corporations and the mass media are all speaking from the same script.
When reality is-biting, it may be denied that this is a bite, the pain can be attributed to some other cause; because there are no 'facts' without interpretation.
Interpretation can deny the factuality of a fact in one direction; or make a lie, or an abstraction, into 'a fact' in the other direction - and people cannot usually tell the difference (nor do they care to try)
Power can make almost anything expedient, at least in the short term; for example believing that men and women can change sex.
This is made true, according to pragmatic definitions, because those who sustain it are rewarded and those who deny it are punished. And that kind of in-your-face reality doesn't go away - at least not in the medium term of years and decades.
And power can - much more easily - make almost anything inexpedient - therefore 'un-true'; even when some-thing is supported by (for example) a century's worth of uncontradicted scientific evidence - such as class or racial differences in 'g' (general intelligence, measured by IQ).
Many things that happened and are real - real according to science, personal experience and common sense - have been made to go away by the combined efforts of education, government, laws and the mass media.
So power can make anything true and anything untrue by PKD's definition - if we define truth according to our practical, everyday experience of things.
This is because there are no facts without theories - and no theories without assumptions.
And assumptions can be smuggled into culture, and minds, either by false arguments or by covertly concealing them in political communications, laws and workplace regulations, news stories, TV dramas, movies etc. CS Lewis called this Bulverism, and it happens all the time, deliberately and strategically - at many levels, and with many twists.
Anyway; if reality cannot be defined pragmatically; then what is real reality; how can we define it?
We first need to assume and acknowledge that reality is really real - we cannot define reality from a place of being agnostic about its existence! The only people who can coherently discuss the nature of reality, are those who acknowledge it is.
We also need to acknowledge that we inhabit a creation. Only if life overall has a reason, purpose, meaning; can we talk in terms of reality - as a term contrasting with not-reality.
The we have to introduce God into the discussion: God the creator. A personal God (not an abstract deity) has to be the creator, if there is to be a reality that Men can know.
Indeed, for men to know - Men must also share in the same divinity as God the Creator. (Which, for Christians, is a fact.)
In conclusion, PKD was writing for a mixed audience of mostly atheists and 'agnostics' (de facto atheists); and was trying to discuss reality in a way that included this audience. But - as we know from the Exegesis - PKD was a theist (a Christian), and he knew from his personal spiritual experience, that materialism, the denial of God and the spirit, could not discern reality.
PKD knew that Man relies upon God (a truthful and loving deity, outside of creation) if Man is to know reality - and this is a theme of much of his best work. For instance; In Ubik, Runciter is 'God', outside 'the world; trying to communicate with Man (inside the delusory and hostile world of cryogenic half-life) by means of signs - and providing help to awake to reality with the substance Ubik.
At his best and deepest - Philip K Dick knew what everybody in 2020 ought to recognise. Without God Man cannot know reality - cannot distinguish reality from demonically-induced delusions, human manipulations, or from wishful-thinking.
And even with God, we still must make the right choices. God-without (the creator) gets us half-way to reality; God-within-us (present because we are children of God) is needed to get us the rest of the way.