You can only trust what is real.
Indeed, when it comes to trust what is real is to be preferred to what is not real, imaginary, or merely apparant. However, there is disagreement about what is real and how knowledge of reality is to be obtained. For John Locke, all knowledge comes from experience. Yet three problems are readily apparant: (1) The senses are known for deceiving and are thus not an absolute source of knowledge. Instead, experience as a source of knowledge is only more or less reliable, probable, but not absolute the way a priori truths are. (2) Consciousness is a source of knowledge, and is not conveyed through any sensory organ. One can, for instance, lose one's sense of sight and still be conscious. (3) Locke is assuming that all knowledge comes from experience; however Kant has demonstrated that certain things can be known priori to experience--namely, a priori truths. For example, a person need not know that if one looks through a rose-colored lens the world will appear rosy. This can be ascertained a priori.
In spite of these shortcomings, or perhaps because of them, rationalists posit that empirical knowledge is next to worthless. Rationalism is a much more reliable epistemological system. It admits no fallacies so to speak, whereas empiricists commit informal fallacies all the time as they make observations and from these induce generalized truths. For rationalists the preference is for deduction. Deductive knowledge is often constructed in the following way. Concise definitions are established that specify a range of phenomena, but only that which is essential. Out of these definitions, axioms are formed, which are self-evident truths that are universally agreeable. From these definitions and axioms propositions are formed, traced to their definitional and axiomatic sources. Propositions are then demonstrated, in some cases by illustrating that the antithesis of the proposition leads to an absurdity and thus the proposition is necessarily true. In total, with definitions, axioms, and propositions, scholars are able to deduce theorems. These theorems will have the advantage of being replicable. In other words, one should be able to follow the reasoning, which is deliberately made transparent, to arrive at the same conclusion now and forever.
Now, when the senses and reason supply you with different and conflicting pieces of knowledge, which do you rely on? Which do you trust?
You might say quite sensibly that you rely on a combination. Certainly this is similar to the idea Kant had in mind when he claimed that the senses cannot think, and the understanding does not see, and what was needed, therefore, was a synthesis of empricism and rationalism. Accordingly, one's best bet is in transcendental logic, where the sensibility is used as content for synthetic judgments a priori.
In short, what is really at stake here is the epistemological system one is using. Mysticism is not to be mistaken for an epistemological system anymore than the tooth ferry is to be taken for something that exists. The reasoning is as follows. All things in the animate and inanimate world take place according to rules. Nowhere is there to be found any irregularity. True, at certain times these rules are not known. The child, for instance, may speak in grammatically correct sentences without knowing the rules of grammar. The nonmathematically inclined person may not see a pattern when given a Fibinacci sequence, but there are rules in each. Both follow algorithms. If all things follow rules, then humans too would have to follow rules. Thought, which follows rules, is in essentials the same everywhere. It is not rue that there are different kinds of laws of thought to suit the different kinds of objects thought about. Even William James Sidis, with his alleged IQ of 300, could not think eleven-dimensionally. Sidis too was bounded by laws. Now, proper thinking is by definition logically valid thinking. If it is not logically valid, it cannot be called proper. For the linguist, the basic laws of logic are necessary for a basic level of communication to take place. For the logician, a basic condition of knowledge is truth and the criteria of truth is fourfold: (1) The principle of non-contradiction (i.e. X cannot be both X and not X). (2) The principle of identity (i.e. X=X and not Y). (3) The principle of excluded middle (i.e. X either is or is not, there is no other option). (4) Principle of sufficient reason. Given this criterion of truth, which as mentioned is a condition of knowledge, it is clear that mysticism cannot meet any such standard. Myticism is the belief in the existence of realities beyond perceptual or intellectual apprehension that are central to being and directly accessible by subjective experience. Such a definition is dubious and cannot even meet the criterion of non-contradiction. If something is beyond perceptional and intellectual apprehension then this precludes the possibility of it being accessible to experience. In order to experience, one must perceive. We don't talk about what we did while we slept for six hours because there was nothing perceived therefore nothing interesting experienced. If, on the other hand, one dreamt then one might have something to talk about. If one slept well or bad, one might also have something to talk about, but only inasfar as the sleep informs the experience when awake because to propound an experience is to perceive and/or discuss it on an intellectual level. Mystics claim to not be binded by the rules of perception and intellectualization. It is my duty, therefore, to tell the mystic that he knows nothing for only in perception and thinking can knowledge be produced. By modus tollens if there is not perception and/or thinking, then knowledge cannot be produced. If knowledge cannot be produced, then this cannot be called an epistemic system. It follows that mysticism cannot be called an epistemic system. Thus, from the point of view of an epistemology textbook, mysticism is intellectually worthless. Therefore, in going back to the original claim,
You can only trust what is real.
I shall agree and go on to note that mysticism is not real; therefore mysticism cannot be trusted.