An idea that would be "dangerous if true" is what Francis Crick referred to as "the astonishing hypothesis"; the notion that our conscious experience and sense of self is based entirely on the activity of a hundred billion bits of jelly — the neurons that constitute the brain. We take this for granted in these enlightened times but even so it never ceases to amaze me.
Some scholars have criticized Cricks tongue-in-cheek phrase (and title of his book) on the grounds that the hypothesis he refers to is "neither astonishing nor a hypothesis". (Since we already know it to be true) Yet the far reaching philosophical, moral and ethical dilemmas posed by his hypothesis have not been recognized widely enough. It is in many ways the ultimate dangerous idea .
Lets put this in historical perspective.
Freud once pointed out that the history of ideas in the last few centuries has been punctuated by "revolutions" major upheavals of thought that have forever altered our view of ourselves and our place in the cosmos.
First there was the Copernican system dethroning the earth as the center of the cosmos.
Second was the Darwinian revolution; the idea that far from being the climax of "intelligent design" we are merely neotonous apes that happen to be slightly cleverer than our cousins.
Third, the Freudian view that even though you claim to be "in charge" of your life, your behavior is in fact governed by a cauldron of drives and motives of which you are largely unconscious.
And fourth, the discovery of DNA and the genetic code with its implication (to quote James Watson) that "There are only molecules. Everything else is sociology".
To this list we can now add the fifth, the "neuroscience revolution" and its corollary pointed out by Crick — the "astonishing hypothesis" — that even our loftiest thoughts and aspirations are mere byproducts of neural activity. We are nothing but a pack of neurons.
If all this seems dehumanizing, you haven't seen anything yet.