# Understanding mathematics imprecisely

For a long time, it has been a complete mystery to me how any of my peers understood any math at all with anything short of filling in every detail, being careful about every set theoretic detail down to the axioms. That's a slight exaggeration, but I certainly did much worse in courses where I attempted to replicate this myself by not reading every proof to save time.

It's only recently, and only within the field of probability theory, that I've developed the ability to do this myself. I am currently following Grimmett's book on Percolation theory. There are far too few details for someone of my level to fill it in completely, but I am getting more than nothing out of it.

Question 1: I would like to learn how to get even more out of such "incomplete" studying.

What tends to happen even now, and more before, is that as soon as I don't understand something, I lose focus and everything just flies over my head. I imagine this is partly psychological, since from a logical perspective if I have to accept proposition $P$ to derive $Q$, I could just think of myself as having proven merely that $P$ implies $Q$, and then no "acceptances" are being made.

Most of the time, professors simply stare blankly at me, wondering how I could persist like this, and all they say is to stop. But it's not that simple, because it appears my intuition is also primarily symbolic. Sure, I think of some geometric pictures when they're called for, but most of my problem-solving creativity comes from pattern matching methods and tricks with situations.

Question 2: How does one distill out the important ideas of a mathematical reading, such as a proof or paper?

Grimmett's book is very helpful in this regard. He will always tell me what's important, and as long as I'm willing to believe him, then I don't have to do anything. But what if I need things that are different than he emphasizes? I always worry that by not understanding everything, I will eventually reach some point in my life when I need to use some fact/method I glossed over and forgot, and that it could be framed in such a way that I would not even be aware of what is missing. That way I wouldn't even be able to do a huge review to rescue the fact from the depths of my ignorance. My current way of thinking about this subproblem of question 2 is that mathematicians always take this risk by not studying everything. So it's a risk-minimization game with time as the constraint. If so, how do I make smart choices with regard to this game?

Question 3: With my recent ability to learn imprecisely in probability, I've started to see many connections, even with outside fields. Many of them are probably fictitious. Many of the questions that I think are highly-motivated might actually be not really worth answering. How does one decide what questions are interesting? As a graduate student who has barely popped out above what the traditional classroom has to offer, I am very lost in this regard.

Question 4: The revelations that enabled me to understand math imprecisely came all at once. A similar comment about the abruptness of my coming upon the ability to proof-check without significant error could be made 2 years ago. Most of my peers seem to learn rather continuously, but the evolutions of my way of thinking seem to come all at once. Is there anything bad or good about this? If so, how do I minimize the bad and maximize the good?

As always, answers to subsets are appreciated.

• What do you think of when you hear the phrase "mathematical intuition?" – Alexander Gruber May 29 '13 at 19:42
• I used to just tune out everything following such a phrase because I would develop my own intuition, typically only after full understanding. Lately, I have even requested specifically that people teaching me in some specific cases say why something is intuitive. I also seek it out myself pictorially, since in analysis, sometimes pictures help more than symbol manipulation. – Jeff May 29 '13 at 19:57
• Terry Tao has a blog post There is more to mathematics than rigour and proofs which seems relevant. – Martin May 29 '13 at 20:26
• I mean this post was mostly about how to become a better mathematician in the sense of what most people think mathematics means, which is less about rigor. So the last comment was more of an excursion into my own personal opinion than something totally on topic. – Jeff May 29 '13 at 20:40
• This is not to suggest I think rigor is everything. I only behaved that way in the past because it was the only frequency I could perceive. But even now, I still think that some of the beauty of mathematics is its rectilinear logical perfection. – Jeff May 29 '13 at 20:46

Full understanding is illusory. If you pursue it, you will find yourself trying to say what a number is, or a set, and digressing into the problem of making language, which for math is a meta-language, precise. And, of course, that can't be done.

So regarding your first question, it might help to observe how futile that innate wish of yours is, and how much you understand without full understanding (or compunction) in all other aspects of your life.

Imagine trying to learn biology and studying the chemical processes in the body, then asking "what is a chemical". You are given an answer that has to do with molecules, a term which you then inspect for precision's sake. Atoms come up, then electrons. Eventually you are learning quantum physics when all you wanted to do was understand how allergens work, or some such thing.

You must operate at the appropriate level for a specific problem. It's no use to reinvent the wheel and do everything from first principles. That would be like writing every program in machine code.

One day, our brains may be augmented with enhancements that allow us to have enough knowledge to understand everything down to our "axioms". Until then, it is a matter of becoming comfortable with our limitations and trying to work with what we have to be awesome.

In terms of knowing which questions are interesting, I think that is one of the harder parts of research. One almost has to be prescient.

And as for getting the important ideas of a proof, my first answer is that sometimes you can't really. Some proofs are just a confluence of numerical estimations and limit results and don't give any real insight into what is going on. Since you seem to be a probabilist, I would point to the proof that a random walk in dimension $n$ is recurrent for $n=1,2$ and transient otherwise. One feels there should be an intuitively understandable reason, but all one gets is Stirling's formula.

For other proofs it is a matter of becoming comfortable enough with the terminology and techniques used in the proof (by re-reading) to see the forest for the trees. In Kung Fu one talks about "learning to forget". You learn the movements carefully so you can perform them without thinking about them when the time comes. You do the same when you learn to integrate or differentiate - you don't want to be doing this from the limit definition when crunch time comes (exam say).

I've observed how my closest colleague did it back in the day of my Phd and postdocs and I took over his technique. What he did was to sit down, read the paper superficially and then try to work out simple stuff he understood on his own. Then he would try to build his own version of what he got from the paper, often without fully understanding what had been going on. But he just had a general idea of the gist of the paper and tried to rebuild the idea in his own words, math, etc...

I remember I then proceeded to do the same later when studying some ecological model that we were trying to pour into mathematical formulas. I felt that the work that had been done was not very rigorous or even incomplete. So I rebuilt the model for myself superficially imitating others at first but gradually abandoning their approach for my own. And this without ever fully learning the necessary techniques of Markov processes, stochastic equations, etc... I feel that by doing this work, my understanding of the material is much deeper than it would have been if I had read a book about it or followed a standard course.

What also helped was the countless conversations and presentations I had to do about my work that forced me to put my thoughts into words understandable to others. They might not have gotten much out of it, but it has been very beneficial to myself for sure.

Sometimes I take solace in:

"Young man, in mathematics you don't understand things. You just get used to them."

- John von Neumann

It seems to me that some of the art is "if-this-then-that" kind of stuff, but there's a whole bunch more that basically comes from the intuition you get from basically just solving problems.

I hope that the original poster will see that this thread is not dead! I wanted to reply to your post, not to only answer your questions, but also to point out where I personally am, and see if you might be able to give me some advice as well (the post is four and a half years old, I'm hoping you've gained some insight by now!)

So assuming you've read the paper by Tao pointed to elsewhere amongst the replies, I will refer to the "pre-rigorous," "rigorous," and "post-rigorous" stages of learning math that Tao referred to, and see if I can elaborate on them with personal (concrete) examples, and relate them to your questions. The first time I "learned" calculus, I completely skipped the chapter on limits. I was learning on my own, from my father's college textbook (he went to school for engineering, so there were plenty of application problems). At this point I was absolutely "pre-rigorous." I didn't really understand the proofs (unless they were purely calulatorial in nature), and I honestly didn't really care (I was like, 13). I did, however, notice that the further I went on, the bigger the gaps were. Eventually, I could not progress, I could not really say I was learning any more, rather just trying to memorize strange formulas. This happened when I started trying to pass from calculus on functions from R^3 -> R, to more general functions (maybe called "vector calculus"). I simply got stuck. Once I finally (and quite recently) came to school, now at the ripe old age of 26, I knew that learning the basics I would have to keep an eye out for the mysteries. I had two goals, I wanted to understand the Fundamental Theorem of Calculus, and I wanted to understand what the hell a Taylor Series was.

There was a moment of epiphany for me, I believe when I was reviewing the definition of a Cauchy Sequence (in R). It was such a drastic, intense thought, I realized that I did not understand any of what I was reading, whatsoever. It sounds melodramatic, but I'm not joking. It was as if there is a switch in my brain, when off I didn't understand logical reasoning, the purpose of definitions, how the material was progressing, and when on I could. I'm not saying that I understood everything at once, but I was able to go back to the beginning and learn it correctly. I don't know if this puts me in the "rigorous" phase of learning, but I believe it did. It was also during this stage of learning that I realized an evil trick played by my analysis teachers. The Hausdorff Separation axiom, as manifest in R, was "proven" at some point early in the exercises... If I ever teach a class in analysis, it will be made very clear early on that this is a particular property of R that is intuitively clear, but not something that is just "true."