> taking on debt for an appreciating (or incoming earning) asset, is generally OK - taking on debt for a depreciating asset is never a good idea if you can avoid it.

this is a good rule of thumb, but there are exceptions. say I want to buy something that costs much less than my net worth, but more than I typically keep in my checking account. I have the choice between a) selling investments, b) taking out a loan, or c) deferring new investments until I've accumulated enough cash.

there are pros and cons to each. with a), I'm paying capital gains tax that I could otherwise defer. with b), I'm taking the risk that my cash flow dries up and I need to sell assets anyway on top of paying interest. and with c), I'm taking the risk that my cash inflates away while I'm saving up.

you can't know with certainty which is best, but b) is usually optimal with a sufficiently low rate.