The quote you're referring to is from the Laṅkāvatāra Sutra, 'things are not what they seem, nor are they otherwise.' It's a baffling saying, as it appears to defy logic. 'Things are not X, but neither are they not not X'. It's an example of what Shigenori Nagamoto calls 'the logic of not' in his essay on Prajñāpāramitā in the Diamond Sutra here.Rachmeil wrote:Things are not what they seem.
Or, more accurate (but klunkier, needs streamlining):
Things are neither what they seem nor not what they seem.
My brief interpretation is that it is a refutation of the customary distinction between reality and appearance. Most other teachings will say: 'things are not what they seem'. And why? They will say: because reality and appearance are different; we are taken in by appearances, and believe phenomena are real. However, in reality, phenomena are simply appearance [instantiations of the Ideal Form/atoms and the void/play of mara] and are therefore unreal. What is real is what is behind or above phenomena. In philosophical terminology, it is the distinction of noumena and phenomena, where the 'noumenal' is the real, and the 'phenomenal' is the unreal.
But the Laṅkāvatāra is saying: appearances are empty, but there's nothing apart from appearances, no higher or real world behind appearance. Hence the saying 'mere appearance'. And that's ultimately because of the non-difference of Nirvāṇa and samsara. The traditionalist believed that Nirvāṇa is radically other than samsara. But Prajñāpāramitā asserts that there's no difference - 'samsara is Nirvāṇa grasped, Nirvāṇa is samsara released' is an aphorism I read once. Two aspects of non-dual reality.
I think it's a very risky teaching, though. It was considered highly radical when it was first introduced and has never been accepted by the Theravada to this day. Worth reflecting on that.