Science is a method of investigating the natural world, and from observations deduce some understanding of how the natural world works. There is conjecturing, and guessing, and dreaming up of stuff — but that is only the first step in each climb up the mountain of the unknown. Following that first step are lots of very non-conjecturing, non-guessing, non-dreaming steps. At any point in this process, given enough minds working on the problem at hand, a little concrete step is formed. And then others climb up that step and start proceeding upwards from there. It is a slow and gradual ascent, built upon the efforts of others that went before.
Conjectures and facts are distinguishable.
I can conjecture that all life is related.
If some bit of knowledge is scientifically established, then you can ask how that is known. If I say that the speed of light is 3×10**8 meters per second, you can ask me how I know that. I will point out to an experiment that measured it — and show that the experiment has been repeated by various people and that the result is verifiable. Repeatable, verifiable, falsifiable — those are adjectives that are associated with a scientific fact and how that fact was indeed arrived at.
That all life has a common origin was a conjecture at some point. That conjecture was provoked by naive observation.
That all life has a common origin was a conjecture at some point. That conjecture was provoked by naive observation. That was then a hypothesis. Based on that, Darwin investigated the natural world systematically. Then he conjectured a mechanism to account for the diversity of life even though the origin is common. That mechanism he called “natural selection.” He did not know anything about genes or DNA or any of the modern scientific facts that relate to biology. His theory got added confirmation when all the later facts were discovered.
So then these are the facts.
We can never be sure that the opinion we are endeavouring to stifle is a false opinion; and if we were sure, stifling it would be an evil still.