Tantra is closely allied to Kriya Yoga, with it pranayam and similar practices which seek to control the inner energies through the breath. The first two stages (yama and niyama) are concerned with the moral life. Controlling this and bringing it into a state of order is the essential precondition for all further progress, for without it the mind can never be restrained. Such virtues as non-injury (ahimsa) of other creatures, truthfulness, purity, contentedness and self-study are necessary. Next come posture (asana), control of the breath and energy flows (pranayama), and withdrawal of the senses (pratyahara). These are not merely physical practices, but stages in the withdrawal of the mind from the physical world which stimulates it and supplies it with its objects. Of posture Patanjali says only that it should be firm and comfortable. The objective is mental concentration, and the many elaborate and difficult postures and exercises developed later by the Hatha Yoga school do not form part of his system. What is much more central is pranayama, control of the prana. Prana is usually translated as 'breath', but it is really much more than this. It is not just the air in our lungs, but the flow of psycho-physical energy which derives from this and, coursing through the living being, finds expression in the activity of the mind and in desires, emotions and actions: 'When there is movement of prana in the appropriate channels, then there is movement in consciousness and mind arises'. In consequence of this, to the extent that the breathing can be brought under conscious control, so too can the activity of the mind. And it is not unknown for the practitioners to fast for long periods, in what is known as Ekadashi. In particular, the systematic lengthening of the pauses between inhalation and exhalation slow down and can even temporarily arrest the mind. Since prana is the life-energy, manipulating it in these and other ways has definite dangers, and it must only be attempted under competent supervision. It is the prana which withdraws from the body at death, and taking with it the other subtle elements migrates to a new body. From the control of the mind, the fifth stage, the withdrawal of the senses from their objects, follows; for it is the mind which directs the senses. This stage is well described in a verse of the Bhagavad Gita.