Scientific American In his 2014 book, Our Mathematical Universe, physicist Max Tegmark boldly claims that “protons, atoms, molecules, cells and stars” are all redundant “baggage.” Only the mathematical apparatus used to describe the behavior of matter is supposedly real, not matter itself. For Tegmark, the universe is a “set of abstract entities with relations between them,” which “can be described in a baggage-independent way”—i.e., without matter. He attributes existence solely to descriptions, while incongruously denying the very thing that is described in the first place. Matter is done away with and only information itself is taken to be ultimately real. This abstract notion, called information realism is philosophical in character, but it has been associated with physics from its very inception. Most famously, information realism is a popular philosophical underpinning for digital physics. The motivation for this association is not hard to fathom. Indeed, according to the Greek atomists, if we kept on dividing things into ever-smaller bits, at the end there would remain solid, indivisible particles called atoms, imagined to be so concrete as to have even particular shapes. Yet, as our understanding of physics progressed, we’ve realized that atoms themselves can be further divided into smaller bits, and those into yet smaller ones, and so on, until what is left lacks shape and solidity altogether. At the bottom of the chain of physical reduction there are only elusive, phantasmal entities we label as “energy” and “fields”—abstract conceptual tools for describing nature, which themselves seem to lack any real, concrete essence. To some physicists, this indicates that what we call “matter,” with its solidity and concreteness—is an illusion; that only the mathematical apparatus they devise in their theories is truly real, not the perceived world the apparatus was created to describe in the first place. From their point of view, such a counterintuitive conclusion is an implication of theory, not a conspicuously narcissistic and self-defeating proposition. Indeed, according to information realists, matter arises from information processing, not the other way around. Even mind—psyche, soul—is supposedly a derivative phenomenon of purely abstract information manipulation. But in such a case, what exactly is meant by the word “information,” since there is no physical or mental substrate to ground it? You see, it is one thing to state in language that information is primary and can, therefore, exist independently of mind and matter. But it is another thing entirely to explicitly and coherently conceive of what—if anything—this may mean. By way of analogy, it is possible to write—as Lewis Carroll did—that the Cheshire Cat’s grin remains after the cat disappears, but it is another thing entirely to conceive explicitly and coherently of what this means. Our intuitive understanding of the concept of information—as cogently captured by Claude Shannon in 1948—is that it is merely a measure of the number of possible states of an independently existing system. As such, information is a property of an underlying substrate associated with the substrate’s possible configurations—not an entity unto itself. To say that information exists in and of itself is akin to speaking of spin without the top, of ripples without water, of a dance without the dancer, or of the Cheshire Cat’s grin without the cat. It is a grammatically valid statement devoid of sense; a word game less meaningful than fantasy, for internally consistent fantasy can at least be explicitly and coherently conceived of as such. One assumes that serious proponents of information realism are well aware of this line of criticism. How do they then reconcile their position with it? A passage by Luciano Floridi may provid