In 1800, William Herschel put thermometers into the colors of sunlight separated by a prism. He wanted to measure their heat. The temperature rose toward the red end of the spectrum, but when he placed a thermometer just beyond the red, where he could see no light, it registered a still higher temperature. He had discovered infrared radiation.
We can discover that our senses have left something out. How would we discover that they had left nothing out, including nothing we have yet learned to ask about?
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By a physics-first account of reality, I mean the claim that a complete physical description would leave nothing requiring any further kind of explanation. An account of what the constituents of the world do would, if sufficiently complete, settle what the world is. I think the strongest challenge to that claim begins before the first measurement, with the conditions under which anything becomes available for us to investigate.
Our questions arise within a world already organized by the kind of creatures we are. Things appear near or far, as happening now or remembered. Questioning those distinctions still requires the capacities we have; it gives us no independent vantage point from which to check whether those capacities leave anything out.
The scientific method corrects for bias by giving error a chance to show itself. When a prediction fails or another investigator gets a different result, we have something against which to check the first judgment. But suppose the limitation acts earlier, on what can appear to us as a question or count as a possible discrepancy. Then the method has nothing to compare. The closure is prior to science, in the conditions that make an investigation possible.
Calling the method self-correcting does not answer this objection. If our way of encountering reality excludes something from every available form of evidence, repeating the test more carefully will not expose it. There would be no result to tell us that anything was missing.
We routinely ask questions about things we cannot directly sense. Mathematics can give us a way to reason about something we cannot picture, and an unexpected result can force a question nobody intended to ask. Science helps us discover how badly ordinary experience has prepared us to understand the world.
Herschel could escape the limits of vision because the invisible radiation had another way to reach him: it heated a thermometer. The result could enter his experience as evidence. That is precisely the opening the scientific method needs. A limit operating across everything available to experience and inquiry would leave no equivalent reading. The discovery of infrared shows how an omission can be corrected when that opening exists; it cannot establish that every possible omission has one.
An unanswered question at least leaves a visible space in our understanding. A question we cannot formulate leaves no such space. From within our present account of things, its absence might look exactly like there being nothing left to ask.
Physics has already taught us to distrust assumptions that felt too obvious to count as assumptions. In his 1905 paper, On the Electrodynamics of Moving Bodies, Einstein examines how clocks at different locations can be synchronized. A clock beside you can record a nearby event. Comparing that event with one beside a distant clock requires a procedure connecting the readings.
We ordinarily speak as though “at the same time” comes ready-made with the universe. Einstein made the procedure explicit. In special relativity, observers moving relative to one another can disagree about whether distant events are simultaneous; the theory specifies how their measurements are related. This is a physical result, not an optical trick or a disagreement about how the events felt. It follows from a framework in which the laws of physics are the same for observers moving steadily relative to one another, and each measures the same speed of light in vacuum.
The “observer” here need not be a conscious person. It can be understood as an inertial frame of reference, a system for assigning positions and times. Clocks can record the relevant measurements without anyone watching, and the theory gives us a common account of their relations rather than a collection of private realities. We have to distinguish what depends on a frame from what holds across frames.
Einstein made an assumption about time answerable to measurement. We learned that something we had taken for granted needed a more careful account. I see no reason to read that achievement as an assurance that every assumption we have failed to notice will eventually become available for testing.
GPS engineers have to account for the different rates at which clocks run in orbit and on the ground. For GPS satellites, the combined relativistic effects amount to about 38 microseconds per day. You should trust the theory when correcting those clocks. A philosophical objection to materialism gives you no reason to omit the correction.
But if you ask me to accept that the theory accounts for everything there is, I want to know how you reached that conclusion. Why should the questions it can answer exhaust the questions there are? Another decimal place in the clock correction will not answer that.
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Someone describes a pain you have never felt. You may understand its cause and believe the description completely while still being unable to supply the experience for yourself.
Thomas Nagel used a bat to make that difficulty harder to overlook. In What Is It Like to Be a Bat?, he asks what it would take to understand experience organized around echolocation. I can imagine myself flying through the dark and responding to echoes, but I would still be imagining what that would be like for me. The bat’s experience is what I wanted to understand.
Nagel does not conclude that physicalism must be false. He asks how an objective account could capture the subjective character of experience, and argues that we do not yet understand how the proposed reduction would work.
Pain does not become a fact only after we have identified its mechanism. If I am in pain, an explanation has to account for what I am experiencing. This is a question we can already ask, not an example of an omission we cannot detect.
I have made my own position explicit: I do not believe materialism can be true. The unity of experience is a central reason for that judgment. How does an account of distributed physical activity become an account of the one experience in which I see this room and hear someone speaking? Giving the activity a more detailed description may be essential to answering the question. Whether it is sufficient is precisely what remains to be argued.
My conviction does not turn that dispute into a settled experiment. But pointing to successful experiments that answer different questions does not settle it either. If someone believes a complete account of the physical activity would explain experience without remainder, I want to know how the explanation works. Saying that it must work because everything is physical assumes the conclusion under discussion.
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When you solve a crossword, the answers check against one another at their crossings. You can finish it correctly without learning anything about a question for which it contains no clue.
Science can change its questions, which a printed crossword cannot. Scientists can discover infrared radiation and, in effect, add a column. But even the enlarged puzzle has conditions determining what can enter it. An error in an answer can be exposed at a crossing. Something excluded from every clue and crossing supplies no such warning.
Suppose physics reaches a theory that correctly predicts every measurement we can make. That would be an extraordinary achievement, far beyond fitting observations already in hand. Does it explain everything there is, or everything that can enter into those measurements?
In The Scientific Image, Bas van Fraassen argues that accepting a scientific theory can mean believing it is true about what is observable, without believing everything it says about unobservable reality. His account of what counts as observable is itself contested. You do not have to accept his position to recognize the question it raises: how much are we committing ourselves to when we say a theory works?
It would be hard to explain such success if the theory were not getting something substantial about reality right. I agree with that argument. Repeatedly predicting what happens, especially under unfamiliar conditions, gives us reason to think we have discovered genuine features of the world.
But a theory could describe real relations with extraordinary precision without fully explaining what stands in those relations. Knowing how two things affect each other is not obviously the same as knowing everything about what they are. Anyone claiming that a sufficiently complete account of the relations would settle that question owes us an explanation of why.
More careful experiments cannot resolve a limitation on what can become evidence in the first place. They can expose errors within what is available to investigation; they cannot independently certify that everything is available. Physics could be complete the way a crossword is complete, with every answer correct inside a field whose exclusions never become visible to the solver.
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None of this gives me permission to put my own beliefs beyond criticism. A limit on one explanation supplies no evidence for another, and my metaphysics has to answer for what it claims.
In 1633, the Inquisition forced Galileo to renounce a position about the physical world. I understand why that history might make someone wary of giving religion any authority over scientific inquiry. But protecting inquiry from religious authority is a different undertaking from ensuring that it always arrives at materialism.
If the objection to opening a metaphysical question is that religion might gain a foothold, we have changed the subject from whether an argument is sound to who might benefit from it. A priest gains no authority over physics because materialism is disputed. Religious claims remain answerable to evidence and argument, as they should have been all along. But materialism must be answerable too. Requiring an inquiry to preserve it as a condition of being taken seriously excludes the disagreement before anyone has answered it.
The possibility of questions beyond our present capacities does not tell us what their answers would be. It does not even establish that such questions exist. A physicist can reasonably say that something incapable of making any observable difference has no place in a physical model. That is a rule about what the model can do. Turning it into a claim that nothing else could exist is a further philosophical argument, and the rule cannot serve as its proof.
A metaphysical proposal has to explain what work it does beyond giving our ignorance a pleasing name. Its testable claims belong under the same scrutiny as anyone else’s. We can examine its premises and ask whether its conclusions follow, even where no experiment can decide between competing accounts.
Herschel found radiation beyond the red because it heated a thermometer. Something he could not see was still able to enter his experience and change what he thought the world contained. I want to know what entitles us to assume that everything there is must have some equivalent way of reaching us. Until we can answer that, the success of the measurements is no reason to declare the account complete.
Go Deeper into the Circus
An Argument in Three Costumes
In June, Geremie Barmé’s China Heritage ran The Longest Relay in its Seeds of Fire series, forty years on from the original volume of Chinese voices of conscience, on the anniversary of June Fourth. The series name is the diagnosis: the Substrate and the Longest Relay
The Bottom Falls Out
Six months ago, in an essay called The World Blinks, I wrote the following sentence about the war that had just begun: “This is going to bring about the end of both regimes. Not because their enemies are strong enough to destroy them — though the accountability is coming — but because the consequences of what they have done cannot be contained within th…