"There is nothing so ridiculous but some philosopher has said it." - Marcus Tullius Cicero
What the heck is reality made of anyway? If you've ever wondered that even in passing, you're thinking like a philosopher. If you were born and raised in the English speaking world, chances are you're some species of empiricist even if you've never taken a philosophy course in your life. You may not know much about philosophy, and your teachers may not, but your teachers' teachers were definitely steeped in empiricism, pragmatism, and behaviorism. I don't get to watch a lot of television anymore, but looking back on it I can definitely see how much it influenced me. In my opinion, modern politics in the Western world owes a lot more to Nietzsche than many people realize.
So why am I talking about this? Our History of Philosophy professor, Dr. Charles James, frequently interrupts his lectures to explain how a certain philosopher or a certain idea influences the world today. I believe the Program for Priestly Formation calls for all seminarians to be acquainted with the philosophical traditions of the country and the culture in which they shall serve. I'll be honest and confess there's a lot in modern philosophy I either don't understand very well, or when I read it I wonder about the author's sanity (we have good reason to wonder in the case of Nietzsche.) Philosophers' writings are notoriously dense. But believe it or not, their ideas still filter into the culture through the universities. Your own grade school, high school, and college teachers may have absorbed ways of looking at the world and philosophical assumptions without even realizing it. If you went to school in the United States, then you've been heavily influenced by the ideas of John Dewey.
Diocesan priests live and work in the world (hopefully without becoming of the world) so we have to know how our flock thinks. We have to be all things to all people, and it's sadly the case that the Church and the modern world are almost speaking completely different languages now. To pose the most obvious question, what would you say to someone who asks you, "Can you explain the Real Presence in the Eucharist? It still looks like a wafer of bread to me." Do you know what "substance" and "accident" mean in philosophical and theological language? Could you explain it to someone who has been raised to only accept as true that which he can learn through his five senses? It's true that some teachings must be accepted on faith because they come to us through Divine Revelation, but we have to use reason to answer objections to those teachings which may come up in RCIA classes or when answering critics of religion in general or Catholicism in particular.
Philosophy is tough, but it's necessary if one is going to tackle theology. And it's necessary to know some in order to understand where your future parishioners are coming from when they ask you questions.