There is a standard sort of exam in microbiology classes called The Unknown. You're handed a numbered tube containing bacteria and instructed to identify it. You get a list of possibilities, but that's it. The path, the method--that's all you.
At the start of my lab, the idea of such a test, frankly, terrified me. I hate open-endedness. Doubt. When all possibilities lay open to me, I freeze, unable to commit to anything. But over the length of the course we were given tools, procedures. A logic took root: if this, then that. Flame the loop. Gram negative or positive? Does it use oxygen, and how much? Flame the loop again.
The Unknown is a test of research, technique, and patience. Results are not immediate; most tests require a sample to be incubated for 48 hours. You can't work too far ahead either--which tests you perform next depend on the results of the test you're doing now. And sloppy technique can set you back 48 hours with inscrutable results.
So you make a key based on your research. You find out what makes each bacteria different--sometimes it's only a single test, sometimes it's ten. What was once an overwhelming list of possibilities is now made manageable. If gram positive and spherical, do a catalase test. If a gram negative rod, check for lactose fermentation. The possibilities narrow, and with every result, certainty sets in. Dichotomy, it turns out, is freeing.
Once you've eliminated all but one bacteria, you have a final step. You test for something known, a result you expect. Confirm what you think you know is right. After that, it's trusting the process: the work and research you put in and your abilities. Do the work, and the unknown is knowable.
Life doesn't hand us a complete list of possibilities. But to do our best with what we know--do the work and don't be sloppy--that's probably enough, most of the time.