Not “mountains” are “mountains”, rather, mountains are mountains
Knowledge can be either accurate or inaccurate; To clarify Zen’s view of what is it that makes ‘knowledge’ knowledge, consider this passage from Dogen on what makes ‘mountains’ mountains:
An eternal buddha says, “Mountains are mountains. Water is water.” These words do not say that “mountains” are “mountains”; they say that mountains are mountains. This being so, we should master the mountains in practice. When we are mastering the mountains in practice, that is effort “in the mountains.” Mountains and water like this naturally produce sages and produce saints.
Shobogenzo, Sansuigyo, Gudo Nishijima & Mike Cross
The difference between saying ‘mountains’ are ‘mountains’ and saying ‘mountains are mountains’ is, first, a difference between words or concepts about mountains and actual mountains and, second, a difference between seeing mountains as they are and seeing mountains as they are not. For example, to see a mountain from the perspective of the representational theory – as an objective independent reality re-presented in the mind/brain – is to see it as it is not. To see a mountain as a coessential element of the experience of a mountain – an experiencer of mountain/a mountain experienced – is to see it as it is. To clearly see mountains, then, is to ‘master mountains in your practice,’ which is ‘your effort in mountains.’ Mountains are ‘what’ (normal; healthy) eyes see as mountains, and mountains are ‘how’ eyes see mountains. In Zen to see mountains as mountains is to naturally (normally) produce mountains as mountains thus to be produced naturally by mountains – hence, mountains ‘naturally produce sages.’