# Having trouble gaining mathematical intuition

I recently started studying advanced mathematics on my own, after taking two classes in college (discrete math and advanced mathematical logic) that spurred my interest in the topic. However, I am struggling a little bit on things that I assume others find easy. I have started a real analysis book (Rudin), and find some of the topics less intuitive and more (seemingly to my brain) arbitrary when compared to the stuff I learned in logic and discrete math.

I specifically feel like I have issues retaining things that I have proved, even if I understand the steps of the proof, because they have no clear intuition in my mind. To give a simple example, consider the following theorem for a field:

Consider $x,y,z \in F$

If $x+y=x+z$, then $y=z$.

The proof for this was fairly easy to me, but I feel like I don't have any intuition on how to remember this. Obviously, this statement is true for the rational numbers, which is a field. But how do I intuit that this is true for all fields? I understand the proof, but don't intuitively understand why its the case by necessity for any field. To me, a field just seems like an arbitrary construct with no real intuition behind it. I feel like I ultimately just have to memorize the result, because without reproving it, I would have no idea whether it was necessarily true or not.

Any suggestions?

Thanks!

• A field is far from an arbitrary construct. Massively simplified, it's the result of someone saying "What exactly is it that makes arithmetic in the rational numbers or real numbers so nice?", boiling down what he thought of as "nice" to an essence of a few simple rules, and giving that collection of rules (along with any construct which follows those rules) the name "field". – Arthur Jun 18 '18 at 14:07
• But isn’t the proof you claim not to remember exactly what you did in high-school to solve an equation? – Lubin Jun 18 '18 at 14:28
• Thank you for your responses! – Paul Blart Jun 18 '18 at 22:16
• Arthur: I guess that makes sense, but I don't know how I'd figure that out. In most textbooks, the definitions are given with little to no motivation. Lubin: The proof of the claim uses the axioms of field addition and is 7 lines. Every result is extremely intuitive when applied to the real numbers or rational numbers, but this is because these sets with the standard definition of addition and multiplication is easy to understand. However, in a field, addition and multiplication can be defined many different ways, so its hard for me to generalize the intuition – Paul Blart Jun 18 '18 at 22:22

The reason is that the field axioms were chosen to be a minimal set of laws from which it is possible to derive the main laws of arithmetic. It is quite possible to understand intuitively why $x + y = x + z$ implies $y = z$ in the familiar field of rational numbers (and even more easily for positive rational numbers), but it's a different matter to pick a voluntarily small number of axioms and start deducing logical consequences from them.
The intuition you should have of a field is that once all of these logical deductions have been done, the familiar rules of arithmetic mostly hold for the four operations $+, -, \times, \div$.
You learn by focusing on the details of the logical development, and by experience with counterexamples, that certain properties true in $Q$ are not true in all fields. For example, $1 + 1 + 1 \ne 0$ need not hold in a field.