Imagine that the scientific community found agreement on a measure of consciousness and selected you—a paradigmatic case of a conscious entity—to test the measure. You are awake and aware while they hook you up to the machine, but, to your surprise, the machine displays, “Zero consciousness detected.” 

Should you trust the machine? Of course not. Whatever the best theories say about consciousness, you require no outer proof to confirm that you are conscious. 

This thought experiment is a modern twist on Descartes’s famous cogito argument: “I think; therefore I am”: even if an evil demon were able to show everything you thought existed to be an illusion, they could never convince you that you aren’t experiencing something. Not only does your consciousness exist, but it is the basis for your knowledge about what exists. In philosophers’ terms, your consciousness is the foundation of both ontology (the study of what exists) and epistemology (the study of what can be known).

Strictly speaking, each of us can only be sure that our own experience exists (0th axiom). Our knowledge about what may exist beyond our experience is inferred from within our experience. In this sense, experience is always primary in IIT. However, for a science of consciousness to get off the ground, we must make some methodological—or operational—assumptions that take us beyond our own experience. In IIT, these assumptions are realism, physicalism, and atomism, explained in turn below. 

Each of these IIT describes as an inference to a good explanation. We say “good” here rather than the more typical “best,” for where should we stand to judge what’s best [1]? However, we can certainly derive “good explanations” in the sense that they allow us to thoroughly account for the regularities we experience (see FAQ: What constitutes a “good explanation” of consciousness, according to IIT?).

If you consider the content of your waking experience, you notice regularities. For example, you’re reading these words on a screen, with other objects in your visual field. This scene could be a hallucination that exists only as your experience, but it seems more likely that something exists independently of your experience. Why? Because if you close your eyes for a little while and reopen them, you see the same screen and same words in highly predictable ways. 

The most reasonable explanation of such regularities is that something other than your own experience exists. In a dream, by contrast, the scenes you experience are often not persistent or predictable—a butterfly might spontaneously morph into a tiger. In other words, objects we experience when we dream do not seem to persist independently of our experience. (In casual speech, we say these contents are not “real”; however, your experience of them is indeed real.) 

The only true alternative thesis to realism would be that your experience is all that exists in a solipsistic universe, but this explains nothing and predicts nothing [1]. We can therefore posit realism: something exists independently of your experience.

Note that we posit realism in the most minimal sense. We merely claim that something exists independently of one’s experience, but we remain agnostic, for now, about what exactly exists and how. For example, the device you’re reading on may or may not exist as “a computer” or “a smartphone,” but its persistence and predictable behavior is at least an indication that something else is likely to exist besides your experience of it. 

Realism alone, however, does not explain the regularities in experience. How can you check whether the piece of paper in front of you exists? The first step is to probe by manipulating it and observing what happens. If you place it further away, it looks smaller. If you push it off the table, it will drop to the floor. The paper “takes a difference” from you and “makes a difference” to you. Moreover, these manipulations take and make a difference in a way that is reliable and persisting. You see the paper drop to the floor every time you push it, and if you do it again tomorrow, it will behave the same way—that is, you won’t see it suddenly turn into a dove instead of simply falling to the floor. Moreover, if you leave it on your desk and return tomorrow, it persists as the same piece of paper as yesterday. 

Hence, in IIT, something exists physically if and only if it has the ability to “take and make a difference”—if it has cause–effect power [1].

Ideally, to assess whether something exists physically, we need to be able to manipulate its state and observe the effects of this manipulation. Imagine you find a remote control and suspect it belongs to a TV, but you don’t see a TV anywhere. You click the power button and then hear the sound of a TV somewhere in the house (you “observe” it with your ears). You can be confident the TV exists because you could both observe it and manipulate it (through the remote). But imagine you click the remote, and nothing seems to happen. What can you conclude? Not much—it may be unplugged, it may have turned ON but be on mute, or it may not exist at all. What exists may turn out to be just the remote (which you certainly manipulated and observed).

Though we must rigorously observe and manipulate to assess the physical, this does not mean we must remain silent about the existence of things outside our reach or capacity for direct manipulation (e.g., distant stars, the core of planets, or the minds of others). For things we can observe but not manipulate, we infer their existence by analogy with things we can manipulate. For example, since we can observe the doppler effect with objects on earth, we can use the same principle to infer the existence of elements in a star’s atmosphere.

In sum, the assumption of operational physicalism stems from our own experience interacting with external entities. But we have no reason not to generalize the assumption to anything that can “take a difference” from and “make a difference” to other things. In other words, anything with cause–effect power exists in a physical sense, and we probe this by observing and manipulating [2].

Note that this notion of physicalism does not presume a “materialist” basis of reality; it only assumes that the physical should be understood operationally as cause–effect power. 

So far, we have assumed that something exists besides our own experience (realism), and that the way to probe what exists is through observing and manipulating (physicalism). But to determine exactly what exists and how, a good strategy is to break larger things down into their smallest constituents and study them in detail independently. This strategy can be called “atomism” (in the original sense of atom, meaning that which cannot be further divided). Only when we have a good grasp of the constituents can we hope to provide an account of how they come together as a whole, without leaving anything out.

Atomism is the core of the “reductionist” approach; we use it even as children first exploring the world. For scientists, reductionism is often ontological: that the whole is “nothing but” its constituents. In IIT, however, reductionism is purely operational: to determine what exists and how, we should decompose the system into its constituents. Ideally, we would observe and manipulate the smallest possible constituents to determine the “atoms” of cause–effect power. But how far down can we actually go? This depends purely on the tools available—on the sophistication of our “atomic eyes” and “atomic hands.” 

As an operational principle, atomism is intended to make our inferences as precise as possible regarding what exists and how. For example, to explain whether and how a tree exists as “a tree,” it may not be enough to merely explain it in terms of the cause–effect power of the roots, trunk, branches, and leaves. Perhaps an explanation starting from, say, the level of the woody and vascular tissue or the sugars produced by photosynthesis would provide a more satisfactory account for what the tree is (if it is anything at all).

Atomism is essential for explanatory completeness; it ensures that we can account for the cause–effect power of the whole based on the cause–effect power of its constituents. Note, however, that IIT’s operational assumption of atomism does not make the theory reductionist in an ontological sense.

Given that experience exists (0th axiom), we can now use the three operational assumptions above as a frame to ask, How can we account for the fact of experience in physical terms? The 0th postulate (also called the existence postulate) guides us to explore this question in purely operational terms—that is, to account for experience in terms of the cause–effect power of a substrate.

The term substrate refers to anything we can observe and manipulate. A substrate could be a set of neurons, which in the context of consciousness science is a reasonable place to begin. But there is no reason to limit ourselves to the brain. 

The 0th postulate employs the assumptions of realism—“there must be something ‘out there’ that is a substrate of consciousness”—and of physicalism—“whatever that substrate turns out to be should be defined in terms of cause–effect power.” The 0th postulate also implicitly includes the assumption of atomism in that a full characterization of a substrate of consciousness must be in terms of cause–effect power “all the way down.”

To assess the cause–effect power of a substrate, we use a perturbational approach: we manipulate units (e.g., neurons ) and observe and record the results in a transition probability matrix (TPM). With a complete substrate TPM, we can then also represent the causal interactions visually using a substrate graph.

The slideshow below illustrates the existence postulate and the operational tools of TPMs and substrate graphs.

As we will see in the coming pages, the 0th postulate puts a strict constraint on the IIT method: it guides us to formulate each essential phenomenal property (axiom) as an essential physical property of the substrate of consciousness (postulate), understood strictly in terms of cause–effect power.

Why not build a theory of consciousness starting from empirical neuroscientific research?

If IIT starts from experience, isn't this method "subjective" and thus unscientific?

What is the meaning of cause–effect power in light of other notions of “causal power”?

Why is the perturbational approach so important in IIT?

How do we get a TPM?