I'm buying shares of an S&P 500 index now with the intention of selling them at a higher price in ~40-50 years. Am I a speculator or an investor?

This may be hidden from you if the index fund (or your broker) automatically reinvests. But either way, you own economically productive assets that a) create value for customers, and b) throw off a stream of cash. Even if the S&P 500 is at the exact same level in 50 years, you'll still gain significantly. That's obviously not true of commodities (e.g., gold, silver) or currencies.