There are lots of patterns in the world. Some of them are governed by the law of gravity, some by other physical principles. And some of them are governed by software. That is to say, the robustness of the pattern, the fact that it's salient, the fact that you can identify it, the fact that it keeps reproducing itself, that it can be found here, there, and elsewhere, the fact that you can predict it, is not because there's a fundamental law like the law of gravity that governs it, but because these are the patterns that occur wherever there are ultimately computational devices, wherever you have organisms that process information. They preserve, restore and repair the patterns, and keep the patterns going. And that really is a fundamental, new feature of the universe. If you went to a lifeless planet and did an inventory of all the patterns that were on that planet, these wouldn't be there. They're the patterns you can find in DNA--those are the ur-patterns, the ones that make all the rest of the patterns possible. They're also the patterns that you find in texts. They're the patterns that folks try to hide by encryption, and that really clever cryptographers nevertheless uncover. These are, of course, patterns to be discovered in the layout of physical matter; they have to have some physical embodiment in nucleotides or ink marks or particles and charges. But what explains their very existence in the universe is computation, is the algorithmic quality of all things that reproduce and that have meaning, and that make meaning.

These patterns are not, in one sense, reducible to the laws of physics, although they are based in physical reality, although they are patterns in the activities and arrangements of physical particles. The explanation of why they form the patterns they do has to go on at a higher level. Doug Hofstadter once gave a very elegant simple example of this. We come across a computer and it's chugging along, chugging along; it's not stopping. And our question is: Why doesn't it stop? What fact explains the fact that this particular computer at this time doesn't stop? And in Doug's example, the answer is that reason it doesn't stop is that pi is irrational! What? Well, the number pi is an irrational number, which means it's a never-ending decimal, and this particular computer program is generating the decimal expansion of pi, a process that will never stop. Of course, the computer may break. Somebody may come along with an ax and cut the cord so it doesn't have any more power, but as long as it keeps powered, it's going to go on generating these digits forever. That's a simple concrete fact that can be detected in the world, the explanation of which cites an abstract mathematical fact about a particular number that is an irrational number.

Now, there are many other patterns like this in the world, which are not as arcane and that have to do with the meaning we attach to things. Why is he blushing? Blushing is a matter of the suffusion of blood through the skin of the face for goodness sake — there's a perfectly good explanation of what the process of blushing is — but why is he blushing? He's blushing because he thinks she knows some fact about him that he wishes that she didn't know. That's a complex, higher order intentional state, one that's only visible when you go to the higher, intentional level. You can't see that by looking at the individual states of the neurons in his brain. You have to go to the level at which you're talking about what this man knows, what he believes, and what he wants.

The intentional level is what I call the intentional stance. It's a strategy you can try whenever you're confronted with something complex in nature ­ it doesn't always work. The idea is to interpret that complexity as one or more intelligent, rational agents that have agendas, beliefs, and desires, and that are interacting. When you go up to the intentional level, you discover patterns that are highly predictive, that are robust, and that are not reducible in any meaningful sense to the lower-level patterns at the physical level. In between the intentional stance and the physical stance is what I call the design stance. That's the level of software.

The idea of abstraction has been around for a long time, and 200 years ago you could enliven a philosophical imagination by asking what Mozart's Haffner Symphony is made of. It's ink on pieces of paper, it's a sequence of sounds as played by people with various stringed instruments and other instruments. It's an abstract thing. It's a symphony. Stradivarius made violins; Mozart made symphonies, which depend on a physical realization, but don't depend on any particular one. They have an independent existence, which can shift from one medium to another and back.