There are many schisms in the vegan/animal rights movement. Perhaps the one that gains the most attention these days is that between the self-styled "abolitionists" and, well, everyone else. However, there is a more fundamental split among the vegan ranks that doesn't seem to be acknowledged often at all.
Vegans can be divided into two groups: a priori vegans and skeptical vegans. A priori vegans take veganism as the starting point of any decision involving animals; a priori is the Latin for "prior to," a term routinely used in philosophy to describe knowledge one "just has" before observation and experience. One striking example of a priori veganism comes from Tom Regan, author of The Case for Animal Rights. Regan, attempting to dismantle the utilitarianism of Peter Singer, cites the fact that, depending on the actual consequences, utilitarianism might not provide a strong case for vegetarianism as one of several reasons to reject it as a moral theory. Regan has decided a priori that vegetarianism (which he uses to mean veganism as well) is correct. Veganism isn't merely the logical outcome of his moral theory; the theory is constructed to justify veganism.
Building a belief system out of one's conviction that veganism is right isn't the only manifestation of a priori veganism. More commonly, vegans will use the definition of veganism as sufficient reason to do or not do some particular thing. Take the anti-consumerist activity known as freeganism. So-called freegans never buy animal products, but will use them if they would otherwise go to waste. A lot of freegans will, for example, go dumpster diving and might eat edible food containing animal products that has been thrown out. An a priori vegan will state without equivocation that this is wrong because vegans don't eat animal products period. Something being "not vegan" is automatically enough to make it wrong. Perhaps it is wrong. But if it is, there is something that makes it so other than not conforming to a particular word's definition.
We can contrast a priori veganism with skeptical veganism. The word skeptical here refers not to skepticism about the goodness of being vegan, but to the traditional skeptical position of requiring evidence to support one's views. The skeptical vegan is a vegan because veganism is consistent with her considered moral beliefs; if, somehow, it became clear that non-vegan activity was consistent with those considered moral beliefs, the veganism would go, not the beliefs. Some a priori vegans may say this makes the skeptical vegan no vegan at all.
Another way of distinguishing these positions is to say that a priori vegans see veganism as an end itself, while skeptical vegans use veganism as a means to some other end. Skeptical veganism can arise very simply from common moral judgments: we ought not cause avoidable harm, animal products harm animals, we can avoid animal products, therefore we ought to be vegan and avoid animal products. Veganism is here the means to the end of not harming animals.
A priori vegans are naturally concerned with not harming animals as well, but they extend veganism to cover cases in which no harm could possibly result. Suppose a vegan comes across a recently dead animal in the deep country. This vegan decides on a whim to eat the animal then and there, and never tells anyone about it. He is clearly not directly causing harm, nor is he contributing to the pervasive belief that animals are property which allows others to cause harm. The skeptical vegan quite probably calls this harmless hypocrite disgusting and may hope he gets diarrhea for his trouble. The a priori vegan, in contrast, calls him immoral. The a priori vegan is perfectly right to say that the hypocrite is not a vegan, at least so long as he keeps such things up. The question is whether being vegan or not actually matters, morally, in cases such as these.
One suspects there are two reasons for the difference between a priori and skeptical veganisms. First is simply commitment. Anyone who devotes any significant time to a cause is subject to a consolidation and solidification of her beliefs. Most vegans begin from a position of compassion for animals, and likely take the simply steps outlined above to arrive at a position resembling skeptical veganism. But over time, veganism becomes so engrained in their behavior and psychology that it becomes easier to simply use whether or not something is vegan as a proxy for whether or not it actually harms animals, and from there it is a short leap to a priori veganism.
The other reason for this division applies more to the theories and theoreticians. The difference between a priori and skeptical vegans mirrors, but is not identical to, the difference between duty-based and consequentialist ethics. Duty-based, or deontological ethics proposes that there are specific moral rules that should constrain our behavior. Consequentialist ethics suggests that whether an act is right or wrong depends on how good or bad the outcome is. Consequentialist vegans are pretty much skeptical vegans by default. But duty-based vegans may or may not be a priori vegans. Those that believe that we simply have a duty not to cause unnecessary harm to animals could be skeptical vegans; there could be fringe cases involving animals that do not cause them harm. But those who believe we have a firm duty not to use animals as means to our ends are likely a priori vegans; even an utterly unharmed animal could thus be wronged if it is being used in some way.
It is probably clear by now that I am a skeptical vegan. I think there are plenty of non-harm-based reasons not to use animal products in most of the hypotheticals above, but "they aren't vegan" isn't one of those reasons. And I would never argue that people should do non-vegan things, merely that they are not always morally wrong if they do. I think taken to logical conclusions, a priori veganism has many silly implications. If using animal products is just wrong, no matter what, the a priori vegan can't use convenience or difficulty of avoidance or even self-defense as an excuse for using, for example, medicine that has been tested on animals. The a priori vegan who avoids certain product brands because they test on animals has no justification for ever shopping at stores that sell meat or leather, or eating at restaurants that serve meat. If the a priori vegan catches household pests and releases them, he can't consistently drive a car faster than five miles per hour for fear of killing insects. For that matter, if we cannot use animals as means to our ends, regardless of if it harms them, even nature photography is forbidden….
But these are absurd. Scratch an a priori vegan, and no matter her conviction, a subconsciously skeptical vegan lies below the surface. A priori veganism is a conscious belief, but rarely one that extends to its furthest implications. A priori vegans can respond by shifting the goalposts: the original definition of vegan, after all, included an "as far as is possible and practical" clause that can be used to justify essentially whatever the a priori vegan wants it to. But note that "as far as is possible and practical" presents a paradox; it's not practical to never take animal-tested medicine or buy from stores that sell animal products or travel in a bug-killing vehicle, but its certainly possible.
But more importantly, a priori veganism is intellectually lazy. Perhaps that's part of its appeal, but it requires no thought and there is no context to consider. Skeptical veganism requires consideration and context. Skeptical vegans avoid animal products where it will probably avoid harming animals or contribute to a reduction in harming animals. Skeptical vegans could go on doing so even if a priori vegans decided to kick them out of the vegan club. And a priori vegans can go on claiming the moral high ground even when it matters not at all.
At least where it does matter, in actual fact, both sides agree.