Of course, it's been discovered that certain plants contain chemical compounds which can stimulate psychedelic (psyche - mind, delic - expanding) experiences when ingested properly under a correctly controlled set of conditions. The most notorious and notable of these is lysergic acid dithylamide (LSD) which has been made seriously illegal by powers with suspect motives and records of venally barbaric and inhuman behavior. Others are alkaloid compounds found in plants such as psilocybin, as well as buds of the peyote cactus (Lophophora williamsii). The two figure prominently in spiritual practices of Native American culture.
For centuries the spiritual leaders of these cultures have employed peyote and psilocybin to move their consciousnesses into other-worldly realms where they commune with, and gain insight from spiritual entities which do not cohabitate with humans in the material realm where exist time and space. As these entities seem to habituate a realm where the imagined woes of material reality do not prevail, they are often mistaken as occupying a superior position in what seems to then reveal itself as a spiritual hierarchy. (This misapprehension of scale can lead to misunderstanding the characteristics of the experience, and inappropriate attributions of power and abilities.)
In the instance of individuals already cognizant and conscious of these other levels of reality, stimulants such as these organic compounds are unnecessary. For these people such experiences gained by their use are already natural experiences to them which require no additional stimuli. This raises the significance of the fact that the difference between the two is that of awareness, or consciousness. Why does the one see when the other does not until using a catalyst? Our primitive condition and lack of any real understanding of consciousness itself looms large as the reason this question cannot be readily answered. Those with this level of consciousness have attempted to answer for those who are without, but those who are without lack the experience (and therefore the insight) to comprehend the answers and insights provided.
Not necessarily well-intending people who claim to be people of science (part of a discipline that has spent an overly amount of its time and attention refuting the religions their own cultures have created) insist that because such experiences as these are not measurable by the standards they have chosen to employ to define reality to themselves (while so doing insisting their standard is therefore the only unassailable factual definition) these forays into the beyond do not exist and are imaginings, or reveries and belief in their significance is merely superstition. They seemingly overlook the fact that a negative cannot disprove a positive. (Lack of evidence is not evidence of lack.) Unfortunately, their lack of objectivity can't help but call their motives into question.
However, this view within a society wishing to define itself as modern greatly distorts in those who hold it any understanding of the self, as well as the nature and significance of experiences such as the ones herein being discussed. More's the pity, in one sense, for the lack of insight and accompanying edification which results. However, it is a freely-made choice to hold this contrary position, and any negative impact is one willed upon oneself. Doing so in the name of knowledge, whilst depriving oneself of knowledge is a contradiction not without its repercussions. Yet, ignorance being bliss, if it matters not to such people, but in fact fills such people with enthusiasm for themselves for holding this view, it would seem futile (and even undesired) to mourn their loss. Unfortunately, their misapprehensions defined as truth oftentimes confuse those who come behind them which does make this a matter for concern. In a fully technocratic society, this knowledge of the self is in danger of being, if not completely lost, at the least permanently hidden away.
Be all that as it may, the salient point is throughout history in many and varied cultures there have always been people who contend they perceive dimensions of reality not available by perceiving only that part of existence wherein is contained time and space. The further contention is then that these perceptions are not made with any of the five senses which gather information from this three-dimensional level of reality (a fourth if one counts time being the accompanying conventional wisdom). It is more than a little interesting that plant substances can augment the perceptive capacities of those who cannot readily perceive these other dimensions. However, all who do perceive these other dimensions as a matter of discipline and course recommend anyone actually pursuing significant access to this aspect of reality should also develop the disciplines required and not rely upon augmentative substances. The obvious question would be, "Why?"
Curiosity in and of itself (and we have to modify this instantly in terms of: What sort of curiosity? What depth of sincerity? With what intentions?) does not seem to be sufficient cause to go to the lengths required to make this exploration. As the social paradigm which governs modern society runs counter to the ethos of the disciplines required to attain this level of perception, the endeavor itself would threaten to alienate the perceiver from conventional society, and quite possibly any means of livelihood this society offers its members. The assumption is humanity has, despite the ups and downs along the way, steadfastly trudged its way to a successful expression of evolution. Should this assumption not be accurate, having it hang over the subconscious efforts of all its members as they endeavor to explore the unknown or the unfamiliar in concepts and beliefs must be seen as sufficient prejudice to make unreliable by definition any attempts (even those which seem positive) to achieve results - lack of objectivity. This would include the belief that the science of physics has achieved such a reasonably complete understanding of the nature of the universe as to have a veto in any conclusions drawn by everyone else. The fact is with regard to consciousness physics has but a scanty awareness with negligible understanding. Their ipso facto conclusion that if they don't then no one else can doesn't rise to require dignifying with a response. (But, this is digressing.)
In order to objectively observe phenomena one must be freed from prejudice of any sort. One must be freed from social indoctrination, and personality imprinting as well. This is why so many neophytes to this make cursory observations followed by uninformed conclusions, then wander away to other things feeling the depths of this are sufficiently plumbed. This on-the-fly wisdom is then related through cliches which inadvertently (or of necessity) become apologetics for the existing social paradigm in which they exist, including values they claim to not share, or religious views they claim to have (by dint of logic) rejected. A sound reckoning of experience would require neither opinion, nor belief. "I looked but I did not see anything," is not refutation. Or, as with the notion of seeing ghosts, "If you don't believe you can't see," cannot be valid. "If you don't believe in further layers of existence as manifestations of consciousness, you can't see them." This is, and well should be, unnecessary and quite faulty to assume. If it exists, it is there, as is said. Inventing imaginings in order to provoke the experience would be needless, and indeed pointless just as one doesn't need to believe in water in order to see an ocean. If you are standing on the shore, it's there before you regardless of what you might believe. How much one allows oneself to see, and what conclusions may be drawn from what is seen are subjective variables the will can alter, or even invent.