What is the test for "arbitrarily" breaching a contract?

Farwell J. in Quinion v. Home, [1906] 1 Ch. 596 at pp. 603-4, defined the term "arbitrarily" (in reference to conditions of sale under a contract) to mean "without any reasonable cause". And with respect to reasons offered by a vendor for his conduct in a case of that kind, he said (at p. 603): "I have to see whether those reasons are a mere arbitrary exercise of his own will, or whether they rest upon any rational foundation on which any ordinary reasonable man is entitled to rely."