The ability to distinguish between fact and opinion, between fact and interpretation or judgment is paramount to successful thinking and writing strategies. In fact, some people would argue that this is what education is really about. In an argumentative essay, it is essential to know what is fact and what is only asserted as fact. What kinds of statements can we make that our readers cannot reasonably dispute?
There is no reason to get excited over someone saying that the American Declaration of Independence was signed on July 4, 1776. If you found some evidence that it was actually signed on July 3, that would be exciting, but historical truths are those that are generally accepted by your readers as common knowledge. Be careful, though, where you push your historical evidence. It is widely accepted, for example, that six million Jews died in the holocaust of World War II (except by some neo-Nazis with unspeakable agendas), but can we claim in our essay, then, that this is the worst display of humankind's systematic cruelty to other humans? Others could point to genocide in other parts of the world; over a much longer period of centuries it is estimated that sixty million Africans were killed in the slave trade. It is wise not to inflate a historical truth into a claim where it becomes debatable. Let the historical fact speak for itself and it will probably do its job quite nicely.
No one would dispute the fact that the depletion of the ozone layer is a bad thing or that the loss of the ozone layer would prove catastrophic to humankind and other living things. People will dispute, however, the evidence that there is, in fact, a hole in the ozone layer that threatens us or even more debatable, they might claim what causes ozone depletion and what measure should be taken to halt or reverse the process. A writer would have to be very careful in citing something as established scientific truth in this area; there are people with political and economic agendas who would love to argue the point.
It is interesting to watch the ebb and flow of what is acceptable scientific evidence. There are very few people who still hold that the earth is flat; in fact, we tend to ridicule those who do. But there was a time when many great minds accepted the notion of a flat world. Although it would be folly to accommodate the views of the Flat Earth Society in our astronomy reasearch paper, a wise writer respectfully allows opposing but legitimate viewpoints their space in his or her essay; it only makes the writer seem that much more reasonable. On the other hand, there is no point in citing scientific evidence in an argument so fraught with high feelings as the nation's ongoing debate about abortion, say. Although we should listen to them, scientists probably have no more right to say "when life begins" than anyone else. The same could probably be said about arguments over euthanasia and when life should be allowed to end.
Generally, a statement based on geography is on firm ground. If we assert that building expensive homes on the sand and glacial till overlooking California's coastline is a mistake, that's something few can argue with. Be careful, though, not to confuse geography with politics. The use of demographics a statistical picture of large groups of people is a lot more shaky than the descriptions of landmass and political borders.
The other sub-sections of this part of Principles of Composition are as follows: