But I hope that our Mars probes will discover nothing. It would be good news if we find Mars to be completely sterile. Dead rocks and lifeless sands would lift my spirit. Conversely, if we discovered traces of some simple extinct life form – some bacteria, some algae – it would be bad news. If we found fossils of something more advanced, perhaps something looking like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life we found, the more depressing the news of its existence would be. Scientifically interesting, certainly, but a bad omen for the future of the human race.
The observable universe contains approximately 1022 stars. The universe might well extend infinitely far beyond the part that is observable by us, and may contain infinitely many stars. If so, then it is virtually certain that there exists an infinite number of intelligent extraterrestrial species, no matter how improbable their evolution on any given planet. However, cosmological theory implies that, due to the expansion of the universe, any life outside the observable universe is and will forever remain causally disconnected from us: it can never visit us, communicate with us, or be seen by us or our descendants.
A von Neumann probe would be an unmanned self‐replicating spacecraft, controlled by artificial intelligence, capable of interstellar travel. A probe would land on a planet (or a moon or asteroid), where it would mine raw materials to create multiple replicas of itself, perhaps using advanced forms of nanotechnology. These replicas would then be launched in various directions, thus setting in motion a multiplying colonization wave.3 Our galaxy is about 100,000 light years across. If a probe were capable of travelling at one‐tenth of the speed of light, every planet in the galaxy could thus be colonized within a couple of million years (allowing some time for the bootstrapping process that needs to take place between a probe’s landing on a resource site, setting up the necessary infrastructure, and producing daughter probes). If travel speed were limited to 1% of light speed, colonization might take twenty million years instead. The exact numbers do not matter much because they are at any rate very short compared to the astronomical time scales involved in the evolution of intelligent life from scratch (billions of years).
And if we discovered the fossils of some very complex life form, such as of some vertebrate‐like creature, we would have to conclude that the probability is very great that the bulk of the Great Filter is ahead of us. Such a discovery would be a crushing blow. It would be by far the worst news ever printed on a newspaper cover. Yet most people reading the about the discovery would be thrilled. They would not understand the implications. If the Great Filter is not behind us, it is ahead of us.
Whether intelligent life is common or rare, every observer is guaranteed to find themselves originating from a place where intelligent life did, indeed, arise. Since only the successes give rise to observers who can wonder about their existence, it would be a mistake to regard our planet as a randomly‐selected sample from all planets. (It would be closer to the mark to regard our planet as though it were a random sample from the subset of planets that did engender intelligent life: this being a crude formulation of one of the sane elements extractable from the motley ore of ideas referred to as the “anthropic principle”.)
Psychoactive substances provide an invaluable tool for studying consciousness. Such substances enable researchers to induce altered states with large effect sizes in a reliable manner. These exotic states are crucial for reverse-engineering the underlying formalism for consciousness. Ignoring them, in our view, is analogous to physicists ignoring extreme states such as black holes, plasma, or supercritical fluids when aiming to understand the nature of energy, matter, and the physical world. These substances are useful for phenomenological research because they enable users to study the subtle structure of their experience with unique clarity.
Traditionally, neuroscience has been concerned with cataloguing the brain: collecting discrete observations about anatomy, observed cyclic patterns (EEG frequencies), and cell types and neurotransmitters, and trying to match these facts with functional stories. However, it's increasingly clear that these sorts of neat stories about localized function are artifacts of the tools we're using to look at the brain, not of the brain's underlying computational structure.