कायेन्द्रियसिद्धिरशुद्धिक्षयात तपसः॥२.४३॥
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‍kāyendriyasiddhiraśuddhikṣayāt tapasaḥ ||2.43||
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‍Through austerity, due to the removal of impurities, perfection of the body and the sense organs is gained. 

Tapas means “discipline” or “austerity”. It also literally translates to “heat” or “fire”. One common meaning of tapas is self-discipline, in the sense of deliberately resisting the natural urges of the body and mind in order to build heat and gain power in a certain aspect of life. For example, resisting the urge to snooze your alarm and instead get up to have a cold shower and meditate is an example of tapas. Resisting the path of least resistance, which is not always good for us, builds heat in the body and mind, and creates power and strength to enforce a positive, healthy habit. This heat burns away the impurities and weaknesses in body and mind and makes us strong, much in the same way that adding fire to clay makes it hard and strong. Tapas is the fire which burns all of the impure desires, greed, hatred and delusion that cloud the mind from realising one's true nature. Thus, in yoga, tapas is highly recommended as the sole catalyst of all spiritual growth. In some way or another, growth only comes from tapas, the internal purification born from the fire of self-challenge, of living out of our comfort zone, expanding our boundaries and opening our awareness. 

In the previously mentioned concept of shreyas and preyas, tapas is the heat, the spiritual power generated by always choosing shreyas over preyas (the challenging, fulfilling options over the pleasant option). Alternatively, by always choosing preyas (the easy path, the path of least resistance), the spiritual heat, the spiritual power is diminished. It leaks out through the hole of indulgence and falls down into the lower centres of consciousness, sexuality, greed and lust etc. Much in the same way that water, when put under heat for some time eventually boils and rises, our spiritual power, when put under the heat and pressure of tapas, also rises into the higher states of awareness, the higher chakras of compassion, creative expression, artistry, clairvoyance and eventually, self-realisation. 

On a physical level, tapas builds resilience and tolerance of the mind and the senses. For example, going one day without food leaves a positive impression in the mind, a positive association that makes it easier to do it again in the future. Once these impressions grow, the mind becomes the master of the senses, not the other way around. Then, even if someone presents us with a lovely meal on our fasting day, the positive impression left in the mind overpowers the pull of the senses towards that object, and we do not take the bait, we remain steadfast in our original impression. As a result, the senses are brought under control of the buddhi , the faculty of discernment that is qualified to guide us. 

Actually, this is just natural law. For example, if we run water down a plank of wood, the water will follow the course of the deepest gorge in the wood. The energy will flow in the most established canal. The same way in the mind, energy flows through the deepest impressions, the ones that have been carved out from prolonged repetition of the same behaviour. Like waking up everyday and sitting for meditation, a deep impression is made in the mind, that becomes a safe haven, a canal for the flow of energy, and eventually it is not at all a challenge to get up and do it. In fact, it becomes the path of least resistance because it becomes the deepest impression that overwhelms the rest. Then, even if the tricky ego comes up with some wayward idea like sleeping in late and staying warm in bed, the thought energy will not conglomerate there, it will pass right over and fall into the deep impression, and, whether the ego likes it or not, you will get out of bed and sit on your mat. This is when spiritual life becomes enjoyable and highly rewarding, when the initial resistance is quelled, and when the tapas flows smoothly. 

Tapas also means “austerity”, in the sense of living a simple, humble, pure life free from luxuries and excess. There are various degrees to how austere one can be. We must find our own comfortable limit. Each person has their own way of practising austerity. For some, abstaining from TV is very austere and challenging. For others, it is not even a thought. For some, holding silence is very austere. For others, it is a joy, not at all difficult. There is no one rule for austerity. Therefore, tapas is a personal, individual aspect of one's own spiritual life. Too little austerity (according to your own measure) is destructive. Too much austerity is also destructive. This was the teaching of the Buddha. He tried the way of full-blown tapas, eating one grain of rice per day for years, sleeping under the elements, becoming a skeleton in the forest for years; all to no avail. Similarly, the lavish life of luxury in his fathers palaces also did not allow him to realise the truth of his existence. Eventually he awoke to the middle-path, the path of balanced living. Not too luxurious, not too austere.

Of course, we must try to simplify our life, gradually, more and more, slowly building up tolerance, slowly building up a liking towards more austere living. One could say, “Oh, I watch 2 hours of TV per day, so I will do tapas and cut it down to 1 hour per day”. Ok, at first, this is good, this is tapas. But, for tapas to really be effective in burning away all of the impurities that have accumulated from aeons of sedentary living, tapas must grow and spread into all aspects of life, like a raging fire. So, we must not stop there. The next step is to stop TV altogether, then stop other things, until gradually, our life is completely devoted to the yamas & niyamas, practising them in all actions and situations. But, with tapas, we must go slowly, otherwise we can get burnt. Tapas is heat remember, and if we play with fire that is beyond our control, we will get burned. If one tries to cut out too much too soon, generally they become depressed and “whiplash-relapse” in the opposite direction. So go slowly, and always practise tapas together with ahimsa, with non-violence, with softness and kindness. For example, if you decide to sleep on the hard floor, forsaking all enjoyment or satisfaction at all, then this is not tapas, it is torture, and it will not bring any positive result. It will only make you depressed and then you will project this depression on others around you. This is not ahimsa (non-harm). This is harming yourself and others. Practice tapas with ahimsa. 

If you feel at all like you are suppressing an urge, then it is not healthy tapas. Suppression means there is a pushing down of the urge, pushing it deeper into our own being, not allowing it's expression, stifling it, holding it in, not accepting it, not embracing it, but pretending that it is not there, not validating it, but repressing it, burying it within. Suppression is not tapas, suppression causes depression and explosion. There is the age-old saying “What is suppressed must be expressed”. You can not deny yourself and the urges that are born from your existence. They just come up and out, and we must pass through them. Only when they have expressed themselves in all their colours, be they beautiful or disgusting, will they then pass, much like a storm passes overhead. So, what this means is that we must embrace the urge and try to express it in a healthy way before we try to renounce it with tapas. For example, one might say, “I am going to give up anger, I will not be angry”. But, if we have that anger inside of us, then it will be suppressed by this thought, in which case it will build up and explode like a volcano. Rather, we should try to express it in a healthy way, like through martial arts for example, trying to get it out of the body and mind, and then resting with it out of the body. Then, any tapas that we do will be smooth and will not result in the formation of any self-admonition, shame, guilt or fear. 

Let us say I decide to give up a luxurious bed and instead sleep on a hard bed, but I feel satisfaction, and even pleasure from the benefits of this choice, then it becomes an example of tapas with ahimsa. The decision is doing me good, it is a positive choice, it brings more joy, peace and love into life. This is appropriate tapas, it generates a heat that deeply fulfils the souls, a heat which validates our existence and brings us close to God. Some people like to eat a heavy breakfast, it makes them happy and then they can spread happiness to others. Some people like to skip breakfast, it makes them happy and then they can spread it to others. Each to their own. The only one common principle of tapas is to gradually increase, until we find the happy medium. And how do you know you have found the happy medium? Well, you will know because you will not be able to go any further. The progress will reach a head. You have found the optimum amount of austerity for your current stage of spiritual evolution. At this point, stay here, don’t push anymore. Just be here, and enjoy whatever degree of tapas you are practising. Gradually, after some time, something will open up, and you might be able to let go a little more, or maybe not! Eventually, like the Buddha, one finds the middle way, the happy medium, and then the active practice of tapas stops and rather that spiritual heat is naturally infused into everyone, without any active, conscious decision.

The Buddha did not have to do tapas in order to maintain his regular lifestyle; it was his second nature; to sleep three hours, wake up, meditate and serve his peers all day, eat a small meal once a day etc. From an external point of view, this is an extreme life of tapas, but for the Buddha, it was his happy medium that arose on its own accord, completely naturally, without any resistance or discipline at all. Within the Buddha, for example, there was no longer any urge of downward-moving, lethargic energy. So for him, there was nothing to fight, nothing to resist. But for us, walking on the path of tapas, there still is lethargic energy that must be resisted, until such time as (due to the realisation of the beauty of an awakened life) it is eradicated for good, in which case there are no more lethargic obstacles (such as sleep and greed) blocking our spiritual progress. As alluded to before, this is when spiritual life becomes smooth and easy, when the tapas is infused into every activity.