In many areas of mathematics, there are problems that admit a natural formulation in any dimension. It often happens that such a problem is easier to solve in dimension $n>k$ as compared to dimension $k$. Sometimes, the problem is solved in all but finitely many dimensions. 


The examples are, e. g., Poincaré conjecture, or Milnor's solution to the "Can one hear the shape of a drum" problem. 


So, the question is, **what is the earliest known example of this situation**? 


One that I can think of is Maxwell's derivation of velocity distribution in a gas, which wouldn't work in dimension 1. But since it's not so clear what the associated mathematical problem is, let's not take it as a cut-off for possible suggestions.



Early examples of problems that are easier in high dimension
Desargues theorem. Suppose you have two triangles $(A,B,C), (A',B',C')$ in the plane such that the lines $AA'$, $BB',\; CC'$ intersect at one point. Then the three points of intersection $AB\cap A'B'$, $BC\cap B'C'$ and $CA\cap C'A'$ lie on one line. And conversely. 


Proof. Lift the vertices $A$ and $A'$ from the plane to the three space. So that triangles do not lie in the same plane now.
The statement becomes almost evident. Then obtain the plane theorem by passing to the limit.


In some sense, there is no other proof: if you want to axiomatise projective geometry in the plane, you have to take this as an axiom.


