Which legal maxim had been applied in the case Hadley
v. Baxendale?

In jure non remota causa, sed proxima spectatur  - In law the proximate, and not the remote, cause is to be regarded. --Bacon, max., reg. 1. (In law, the proximate, and not the remote, cause is regarded.) The maxim is chiefly applied to cases of marine insurance, as to which it was held by the House of Lords in Dudgeon v.Pembroke, (1877) 2 App Cas 284, that any loss caused by perils of the sea is within the policy though it would not have happened but for the concurrent action of some cause, as un-seaworthiness, which is not within it. The maxim is also frequently applied to measure of damages, as to which see Hadley v. Baxendale, (1854) 9 Ex 341, where it was laid down that only such damages are recoverable for breach of contract as (1) arose naturally from the breach itself, or (2) might reasonably be supposed to have been in the contemplation of both contracting parties at the time of the contract as resulting from breach.