I'm a philosophy minor, who took first-order logic (philosophy department, not the math side) and I have a chance to take modal logic next semester (also under philosophy). It's going to be a much smaller class and will likely be harder. But it's also essential to doing work in and/or understanding some areas of philosophy (for example, philosophy of religion's ontological argument for God's existence is based on an argument in modal logic). In trying to gauge whether it would be worth my time, I'm wondering how often modal logic (and/or first-order) is used in math subjects? Thanks.