How can we know whether phenomena are real?
Firstly, we must ask ourselves what will change if we stop believing in some phenomenon. If we have no particular reason to believe that the phenomenon will cease to exist when we stop believing in it, then that phenomenon could possibly be real.
Universality
But we may be deluded. Therefore we must go one step further. We must consider what it is that characterises things that actually do go away when you stop believing in them. What is it that makes for something to be an illusion, or perhaps a delusion? Any phenomenon with those characteristics we must suspect of being “not real”, even if other aspects of it lead us to believe that it will not be affected by our suspension of belief in its existence. We must consider why we have ever reached false conclusions as to the reality of things, and demonstrate that the same causes are not applicable to whatever it is that we claim is real. One source of error lies in our psyches. Once we have convinced ourselves that something exists, it can take inordinately much countervailing evidence to convince us otherwise. Therefore the universality of the belief in the existence of a phenomenon matters, for we know by experience that if we can see something, but nobody else can, then it is quite possible that its existence is dependent on our belief, particularly if there is no compelling reason why others should not be able to see it.
Sensory deception
Another source of error lies in our senses. Our senses allow us to perceive and measure. However, our senses often deceive us. Books on psychology are full of examples: lines of the same length which our senses tell us are different, or the other way round; areas of the same colour which our senses tell us are different; even gorillas moving between the players on a basketball court without our senses registering them. We know by logical deduction that if different ways of measuring the same phenomenon produce contradictory results, then, somewhere along the way, our belief systems have led us to interpret at least some of the results incorrectly, and what we mean to see is at least in part an illusion. Conversely, the greater the number of observers, measurements, times and places involved, the greater the diversity between individual instances of these, and the greater the agreement between them, the greater our confidence may be that the phenomenon is real. That applies even in cases where our senses delude us, because even when one of our senses delude us so consistently that we could be fooled, our other senses don’t join in; delusions do not manifest themselves consistently across all senses. Or maybe they do, but then the maxim applies that if we can’t tell the difference, it doesn’t make a difference. Therefore we may regard the things we collectively and consistently see, hear, taste, smell and feel as real, once we have corrected for such delusions.
Modelling errors
A third source of error lies in the way our brains make sense of our observations. When we observe something, we make a model of it in our brains. When you stub your toe on a rock, your brain will probably record the event as a combination of the ideas “toe”, “Pain” and “hard object, possibly a rock”. It is hardly possible to do this without pre-existing conceptions of these ideas. Much of the mental development we go through in our childhood years, starting in the womb, is concerned with developing such conceptions. That process is essentially concerned with developing a model of ourselves and our environment. Our observations cannot be any more correct than the mental models with which we represent them. But these models are the product of our own development, which is different for each of us. For some, this has led to the conclusion that we cannot know what somebody else means when he says that he has stubbed his toe on a rock, because we cannot access his brain. Whether or not this is true is not relevant to our conception of reality, because we may, by a process of asking him to tell us what he means, come as close as we wish to the assurance that he means the same things. We may therefore conclude that shared mental models bring us closer to objective descriptions of reality.
Of course, even shared mental models can be false. A mental model is a human construction. It comes into existence when somebody invents them and stops when the last person to think them dies. They may correspond to something real, in the way that the word – or, to use Plato’s terminology, the form – ‘rose’ corresponds to a flower having the particular characteristics which we associate with roses. Or they may not, for example the form ‘unicorn’. If any reality lies behind such forms, it does not come into existence when we start thinking them or go away when we stop. Whether we believe in unicorns or not has no effect on the number of unicorns on our planet. But is there then any essential difference between roses and unicorns? The answer, fortunately, is “Yes”. The form “rose” has predictive powers. For example, it tells us that if we have a plant which produces a rose, then all other flowers on the same plant will also have the characteristics which we ascribe to roses. Because these predictions are successfully confirmed, not only by ourselves but also by other agents at other times and places, we may be confident that there is something which we call a rose. The same does not apply to the unicorn; we have no record of this form ever having led to any confirmed prediction. Some models cannot even be used to make predictions: conspiracy theories, in which any stray fact is taken as proof of the conspiracy, and any normal fact as proof of the lengths to which the conspirators go in order to prevent us from discerning their conspiracy, are a case in point.
Limitations on corroboration
That predictions are confirmed is never a proof of the mental model. For example, the motions of stars and planets were successfully predicted by the Ptolemaic system for more than a thousand years. In the Ptolemaic system, the sun, stars and planets all revolve around the earth, and a system which employed circular epicycles to modify the planetary orbits and varying planetary speeds along the epicycles predicted the observed motions well enough. It predicted them better, in fact, than the heliocentric model of Copernicus; it was only when Kepler introduced elliptical orbits that the heliocentric view of the solar system improved on the accuracy of the Ptolemaic system. That a model works does not prove it to be true, but is a strong indication that there is an underlying order in the phenomena of which it is a model. No order and regularity can be sustained across all places and times without some underlying cause. The Ptolemaic system is an example of a contrived model: it was good at explaining the observations on which it was based, but never successfully predicted the results of new methods of observation. In order to fit the results of more accurate measurements, the Ptolemaic model always needed to be elaborated. We must be ware of such models, because they tell us more about our ability to contrive models than of the reality which they purport to represent. We must also be careful about confirmed predictions. Sometimes we see what we expect to see, even if that which we are observing is different. For example, there is a notorious experiment in which American whites were shown a film in which a dark-skinned man was attacked by a white man and were then asked who started the fighting. The vast majority said that the dark-skinned man started it.
Some such mental models never spread further than a small group, because different people expect to see different things. But history is strewn with models which have built on shared expectations in such a way as to have fooled all of the people some of the time. Although shared conceptions are more likely to be free of individual delusions, they can create delusions of their own. Given that the world's religions differ so much that at most one of them can be right, all of them, except perhaps one, produce collective delusions. These delusions can persist because the religions include collective taboos on asking those questions which, if properly addressed, would initiate a process in which the religion would be unmasked. Only a belief system which has no taboos can claim to be free of delusions.
Science
Science aims to be such a belief system. It is a systematic approach to understanding the universe by means of testable explanations and predictions. It is carried out by a worldwide community of scientists, within which there is such a diversity of cultures and thought systems that biases to which an individual scientist may be prone are likely to be challenged by others. Test results that cannot be reproduced are rejected.
However, as the history of science demonstrates, even scientists are subject to collective delusions, which get in the way of them seeing things as they really are. Fortunately, there is one class of models for which we need have little concern that the expectations of scientists affect what they see: the models that make seemingly absurd predictions. Nobody expects absurd results, so it is extremely unlikely that observations that confirm them are in any way influenced by our expectations. Such models are, arguably, the closest we can get to the truth.
Quantum mechanics
The theory of Quantum Mechanics fits the bill of making absurd predictions. It was developed early in the twentieth century in order to explain how matter and energy behave at the level of the very small. In particular, it describes how matter and energy can behave both as waves and as particles. Photons, electrons, atoms, even complicated molecules all behave exactly as Quantum Mechanics predicts. It is valid not only for single particles, but also for combinations of particles – physicists call them quantum systems - which interact with each other. All its predictions – even the seemingly absurd – have turned out to be valid. There is no scientifically documented evidence that appears to contradict the theory. It is the most accurate and reliable theory ever devised by science. Where it conflicts with General relativity, experimental evidence consistently confirms that Quantum Mechanics is right and General Relativity is wrong. In other words, of all scientific theories it is the one for which we have the least cause to doubt the reality of the picture it paints of our universe. It is for this reason that the Quantum Occam hypothesis is grounded in Quantum Mechanics.