The second and also very important principle of Yama is Satya.
Let’s look into its meaning. Satya means “real” or “reality”, and “true”, “truth”, “truthful”. Nowadays some people explain this principle as “to be honest and speak only the truth”. But the meaning of Satya is much deeper and more complex for ordinary people – because one cannot know the Truth if he hasn’t experienced himself as eternal Atman. One usually knows only the relative truth when he perceives that part of Reality which is connected with him. And don’t forget about the role of the mind which always affects feelings and perceived information.
Even though we may possess the knowledge of a real fact we would like to share, it is important to remember the first principle of Yama, Ahiṃsā. Our Satya should always consider Ahiṃsā, because words can harm or even kill. We need to foresee what our truth can do to the person to whom it will be told. If it is going to help that person to evolve, even by putting him through a painful situation, then that is wonderful. But often we can have another scenario when our true words lead the person to depression or uncontrolled actions, increase the pride or other vices. That’s why Satya is always about harmony with the current situation.
The word Satya originates from sanskrit root “sat” that means, according to the Monier-Williams dictionary, “that which really is, entity or existence, essence, the true being or really existent”. So Satya requires a really deep understanding: it is about our true nature, about our existence in this Universe. The point is how we express ourselves. Expression is three-fold: thoughts, speech and actions. These can be true, sincere, harmonious, or distorted. In this respect, honesty towards one’s self is important. It requires a certain amount of courage because we will have to admit to ourselves some not very pleasant things. When you gradually increase the level of your truthfulness, you become capable of thinking and acting in a more harmonious way, and as a result right energy exchange with the world sets in. Lies require more energy to be sustained, they also require some false (non-harmonious) structures and energy doesn’t flow freely.
So we can see the practice of Satya as a way of self-knowledge, cognition of our true nature that is the aim of Yoga. By being truthful in thought, speech and action we bring real purity into our being and our practice.
I feel that Satya is really the most difficult Yama and that's why its' perfection can advance someone on the spiritual path for good!