I definitely think you can make good beer without any tools, but I haven't done a 10 year apprenticeship to figure out when the water is the right temp for mashing
I was teaching a guy who had just finished cullinary school how to brew. When I said I wanted the sparge water to be at 170F, he looked at it for a while then said it wa, at 170F. I was on the side with the bulkhead thermometer, and it was 170F. WTF? I asked him how he knew. He said if you pay attention, water as it is heated will have some different character. with temp. At 170F the surface becomes very still and even. Look sometime, I was convinced that the brewers before instuments had some clues as to what to do.
To add to this a little more, he said that as a professional chef, you can't wait around taking temps, you look and do, as there is no time to waste.