Arithmetic

C. F. Gauss, generally considered the greatest mathematician, published his Disquisitiones Arithmeticae when he was 19 years old. The book is still worth studying. A faculty for numbers will get you through it.

From Excursions in Number Theory by Ogilvy and Anderson.

In the beginning there were no numbers; or if there were, primitive man was unaware of them. Whether the numbers were always "there" (where?), or had to be invented, has been a much discussed question, and we shall leave it to the philosophers to continue that discussion without our aid. What we can say with some assurance is the the ability to count came relatively late to civilization. Nineteenth-century naturalists claimed that some animals could count up to 5. Early man could not do as well, and there are known to be isolated primitive tribes even today where any quantity more than 3 is known simply as "many."