Fixity is a key concept for digital preservation, a cornerstone even. As we’ve explained before, digital objects have a somewhat curious nature. Encoded in bits, you need to check to make sure that a given digital object is actually the same thing you started with. Thankfully, we have the ability to compute checksums, or cryptographic hashes. This involves using algorithms to generate strings of characters that serve as identifiers for digital objects. Under normal non-tampered with conditions, these hash values more uniquely identify files than DNA uniquely identifies individuals.  When we generate these hashes for digital objects to audit digital content we want to know if an object is the same as it was before.  Is it still bit-for-bit the exact same thing?  It is important to note that the “is” in that last sentence is only one tradition of saying that something is still the same.

Recording of single magnetisations of bits on a 200MB Harddisk-platter; Matesy GmbH

An analog corollary to this kind of fixity checking is helpful in unpacking the different ways we can say “this is the same thing”. To ensure the authenticity of copies of texts  scribes would count their way to check and make sure that new copies had the same middle paragraph, the same middle word and the same middle letter. It’s an analog fixity check; a technique to check if the encoded content of the text is identical to the encoded content of the copy (functionaly, it is a rather poor fixity check, but a fixity check nonetheless).  In this case, much the same as in computing, the two scrolls would have the same text on them but they are actually two physically different objects, potentially created by different scribes and expressing unique characteristics, for example, each scribes handwriting. If you had two copies of the same ancient text and you told a manuscripts specialist they were identical they might scoff at you. Clearly they are two different artifacts; they are two distinct material objects that have their own physical properties. If we looked into  the chemical properties of the papyri that each was encoded on we might be able to date them and find out which one is older, or we might find that the materials of one came from one place and the materials of the other came from another. While the encoded text of the two objects could be identical, there is an infinite amount of contextual information that could exist in the materiality of the objects they are encoded on.

The is of the Autograph and the is of the Allograph

Is means different things in different statements. This is Mary Shelly’s Frankenstein and this is the Mona Lisa. (Ok, so those are links to Frankenstein and the Mona Lisa). However, the  link to the Mona Lisa isn’t really a link to the Mona Lisa. The Mona Lisa is on the wall in the Louvre. That link just points to an image of the Mona Lisa.  If you load up the link to Frankenstein and the image of the Mona Lisa you can think through two of the different ways that something can be the same as something else. Most would agree that the former is  Frankenstein, but that the latter is a copy of the Mona Lisa. Something is Frankenstein when it has the same text in it. In the art world, these kinds of art are referred to as allographic. You are actually looking at the piece of art when you see something that has the same spelling, that has the same encoded information. It is the same thing when it has the same encoded information in it. In the case of the Mona Lisa, we demand a different kind of is, the autographic is. There is only one Mona Lisa, it’s on the wall in the Louvre.

These conceptions, of something being the same as something else have corollaries in how Matt Kirschenbaum defines assertions that digital things are the same. In his vocabulary there is a formal sense, in which one object has the same bits as another (the same one’s and zeros) and the forensic sense, in which we think about how those bits are physically encoded and inscribed on an individual artifact. All the bits we care about are inscribed on storage media. Interestingly, in the forensic sense, all digital objects are also analog objects. While we read bits off disks each of those individual bits is on some level its own little unique snowflake. Each bit could conceptually be analyzed at the electron microscope level as having a signature, as having a length and a width on the medium on which it is encoded. This said, there really aren’t many cases in which we care about the physical material sense of the forensic bit. Sure, it is possible to use forensic techniques to read back several reads on a hard drive, but even in that case, what we care about is reading back layers of the encoded information, not examining the qualities of the actual bits themselves.

The Mutual Exclusivity of These Senses of Sameness

I find it interesting that these two different senses of sameness, the allographic and the autographic are fundamentally mutually exclusive properties. Try this little thought experiment. Imagine someone came up with a way to compute a fixity check on people. It might look like a CT scanner or something. It would scan you and then generate a string of characters that more or less uniquely identified you. If you came back the next day, climbed up in the machine again, and got your next reading your numbers wouldn’t match. Our bodies are always changing, today I had a lot of coffee so I have more caffeine in me, tonight I might go to spin class and then as a result tomorrow I would have burned some calories. This isn’t just the case for living things. Entropy (and it’s step-cousin in conservation science inherent vice) explain to us that all objects are in flux, slowly deteriorating toward the ultimate heat death of the universe.

Imagine if we stuck some fantastic rare book in this device that checks the fixity of physical objects, how about the Library of Congress copy of Sidarius Nuncius (not these digital images but the actual physical book). Even here, if we came back the next day we would get a different string of characters. While conservationists do their best, from day to day there are changes in things like the water content in pages or other minor fluxuations in the chemical composition of any artifact. I suppose if the device wasn’t particularly sensitive it wouldn’t detect the difference, but even if it did say they were the same thing we would know that it was a lie, it just wasn’t sensitive enough to pick up the subtle changes in the artifact. This is a key distinction between analog and digital objects. Digital objects are always encoded things, in this sense they (like the text of Frankenstein or the text transcribed by scribes)  are allographic. Their essence is actually more allographic than those analog corollaries, as the encoding is much richer and leaves much less interesting information residing in the artifact itself. The medium on which a text is inscribed and the autographic components of an individual scribe or printer’s work actually carries a lot of interesting information in it. In contrast, a forensic disk image of a hardrive contains considerable information about the size and nature of the medium (the drive) and the additional information beyond the bits on a drive is actually older bits (computer forensic folks can get previous writes of a disk by looking at the parts where the write bands overlap.)

What is wild about digital objects is that there are extensive forensic, or artifactual, traces of the media they were stored on encoded on inside the formal digital object like a disk image. That is, the formal object of a disk image records some of the forensic, the artifactual, the thingyness of the original disk media that object was stored on. The forensic disk image is allographic but retains autographic traces of the artifact.