If one practices śamatha meditation without vipaśyanā, one will not be able to understand the true nature of phenomena; one will just be able to rest the mind on something. It is like being on a vacation; one experiences peace on a vacation, but one does not get any lasting results from it.
If you practice vipaśyanā without śamatha, you will not be able to eliminate whatever negativity needs to be eliminated, because vipaśayanā without śamatha is unstable. So even if you have the understanding of vipaśyanā, your mind will be agitated. Therefore you need to have both śamatha and vipaśyanā.
from the book The Practice of Tranquillity & Insight: A Guide to Tibetan Buddhist Meditation
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