In his book, The Number Sense: How the Mind Creates Mathematics, Stanislas Dehaene explores the research behind our understanding of numerosity. He looks at research over the past 100+ years that suggests animals have an innate understanding of numbers. He then goes on to explore and explain the acquisition of number sense in humans from infancy to adulthood. What is our innate capacity to grasp the abstraction of what numbers mean? It is a fascinating and comprehensive look at this question, especially as it relates to how our education system is designed to educate students in mathematics. Here is a link to a video interview (English subtitles) with Stanislas Dehaene.