Both, or neither?
I look at the last call it 5 years earnings just to get my bearings on the business. Then I look forward - are prospects better, worse, or similar to the past few years?
If I think the prospects are way better (something like Fairfax in 2021), but the market is pricing as if prospects are middling, well maybe there's an interesting investment.
Ultimately it comes down to what the business will earn in the future, vs. what you pay today. We know today's price, the rub is in estimating future earnings.
I think prior earnings history does provide some baseline at least to start estimating.