Raymond Tallis ponders hallucinogens:
Their power to terrify may be in part due to their content, but even the most benign hallucination is deeply unsettling precisely because, as [Oliver] Sacks says, there is no “consensual validation”. Nobody else can see, hear, feel, smell or taste what you are experiencing. To be in the grip of such incorrigibly private experiences, adrift in a world populated with items that others cannot confirm, is to be sequestrated in the most profound solitude. Even before we speak, we will point out things that we see and desperately want to share with others. Joint attention to items that we all agree are before us is the basis of a common human world. The involuntary perceptual dissidence of the one who hallucinates reminds us how frail and transient is our occupancy of this world; and how, even when you and I are side by side in the sunlight, each of us may be sealed in the privacy of our minds.