> I increase my liabilities (e.g. by paying by credit card) by subtracting from the account?!?
Maybe I'm just inured to the convention, but this makes perfect sense to me. You subtract money from any other account you pay from. At a sufficiently abstract level, paying by credit card is the same thing as overdrawing your checking account. You need to add money to get the balance back to zero.
Makes perfect sense to me too. Credit cards aren't money you have, they're money you can borrow. Paying by credit card is borrowing money. Paying a credit card back means it should show in your system at $0. If your bank shows it at some positive $, that's just convenient financial fiction.
(And possibly dangerous fiction. I think a big part of the way people get in trouble with credit cards is by thinking their credit line is "the money they have", instead of "money they can borrow really fast".)
'0xcde4c3db makes a good point about it making the flow of value clear in the transaction, but the transaction view is already the easy thing to interpret. The value (heh) of bookkeeping is when those flows are aggregated, and the meaning of those aggregations is (admittedly only somewhat) obscured when optimizing for clarifying flows in single transactions.
Charging a credit card to make it more negative does match our intuition of how it affects our overall financial position, but that's due to serendipity. What does it mean that expenses are normally positive, is that a good thing then? Or that my paycheck account is negative?
In the end it's a nit, a small thing I mostly ignore. I just think that part of the complexity it tries to eliminate is essential, with results like negative revenues. It's a leaky abstraction.
It does work for modelling flows, but the meaning of balances gets a bit wonky. Income is revenues - expenses in the debit/credit model, but in the plus/minus model it's -revenue - expenses (hledger's income statement report flips the sign on revenue accounts, so you get the net via the normal equation).
A different way to do double-entry-lite might be to retain plus/minus, but have that operate on the account's normal balance -- pay for expenses by decreasing an asset or increasing a liability. Transactions still balance even if they don't sum to zero -- it makes sense that an IOU balances out the coffee you just got. You do need to understand the four basic account types (explicit equity accounts are arguably not relevant to most individuals), but that's something that's needed anyway. You still don't need to explicitly learn about debits and credits, but if you do it's much closer from here than from the flow-based approach.
Flow is easy, but I feel that in the end modeling the effect on balances is simpler.
Maybe I'm just inured to the convention, but this makes perfect sense to me. You subtract money from any other account you pay from. At a sufficiently abstract level, paying by credit card is the same thing as overdrawing your checking account. You need to add money to get the balance back to zero.