And in The Heart-Essence of Vimalamitra, Longchenpa says,
Buddha will never be attained on the paths of the nine graduated approaches by engaging in their view, meditation, and conduct. Why not? Because in the views of the nine approaches, there is only intellectual conjecture that is sometimes convincing and sometimes not, but which can never induce the naked essence.
In this context, Pema Rigtsal explains, This pure presence is primordially free of conceptual elaboration and is the contemplation of the minds of all the buddhas. Putting any effort into purifying it or adulterating it by concepts tends to conceal its nature and is counterproductive. We need to abandon all effort, along with deductive reasoning and speculative concepts.
Shamata meditation technique is not exempt from this blanket rejection of all contrived meditation methods:
During formal contemplation, gross happiness and sadness will not arise, but when we get up from shamata, the joy or the pain will come as before. Just as we contain a heap of dust by sitting down slowly on it, but upon our getting up, the dust arises in clouds, in the concentrated absorption of child’s play, gross thoughts are stopped for a while, and we seem to experience happiness, but when we arise from the concentration, we find that more gross thoughts intrude than before.
Shamata, concentrated absorption, does not induce recognition of the nature of mind, but it can provide a relative calm in which to appreciate the profound refinements of cultural Vajrayana. Indeed no cause or condition can make manifest the realization of pure awareness as the ground from which all causal phenomena arise.
Keith Dowman commenting upon Tulku Pema Rigtsal's text