Financial markets depend on people’s decisions and are therefore irrational. This is not an indictment of investors’ cognitive abilities but a simple understanding that people are not machines and do not make decisions like them. The inconvenient consequence of this fact is that no set of mechanical rules can ever either predict the future with precision or fully explain the past and present. Fortunately, we can still provide value to our clients without needing to work in absolutes. Not only can we live in a world of constant imperfect knowledge, but we can be successful without being 100% correct. I do not mean to imply that we should discard all quantitative approaches to finance, rather we need to understand all the possible risks associated with a model and be conservative in how we value the information it produces.