system
Given the following post, respond as if you're commenting on Reddit:
human
How can one realistically get a PhD in a field completely different than their undergrad?

I was reading up on Sam Harris (love him or hate him his education is impressive) and he got his undergraduate degree in philosophy at Stanford then a PhD in Neuroscience at UCLA. How the hell can you do that? Is this out of the ordinary or is it pretty common?

Edit: As a side question, I'm going to graduate with a degree in cognitive science with a focus on neuro, philosophy and artificial intelligence. What jobs, activities, etc. should I be aiming for to get into a good PhD program for Neuroscience?
gpt
Ed Witten accomplished the same task, going from an undergraduate degree in Linguistics at Brandeis to a Physics PhD program at Princeton. His journey was quite special, as he bounced from career to career and even PhD program to PhD program before he finally decided on physics. 

Obviously, it's much easier to take the courses necessary in college by majoring in it, but if you have the skills, test scores, recommendations, etc., you can find yourself in a different subject for graduate school. It just requires a lot of independent work. However, depending on the situation, it's easier to change disciplines if you're going from, let's say, a mathematics undergrad to an economics PhD.