[MR4D]

What if I buy crude oil in Texas, where it’s cheap, with the idea of selling it where it’s expensive (say, Europe) ?

I’m still investing.

In fact, I could build a company around it. And if I did, and then sold stock in that company to you and you used that as investment for retirement, then you would have just validated that buying a commodity is indeed investing.

Maybe not all the time, but certainly some of the time.

By the way, you can buy some of these companies today, under tickers such as TNP, NAT, DHT, among others.

[brokensegue]

investing implies a long term horizon. if you are just taking advantage of an arbitrage opportunity then you are a commodity distributor/shipper or commodity trader. you aren't investing in the underlying asset you want to hold the asset for as short a period as possible to avoid the arbitrage disappearing beneath you.

[MR4D]

Investing implies no such time horizon.

Investing is spending money with an expectation of a return.

For contrast, expenses have no expected return.

Neither have a time horizon. And further, what would “long term” be anyway? Not sure how you could ever define it.