The tortoise is only aware of the water surrounding it. It is ignorant of the renewal of water around it because of the movement of the current. We are hardly aware of the movements of perpetually flowing natural currents of matter around us. These currents are not merely flowing through the human bodies, but are engulfing all beings and even flora and other material objects regarded as inanimate.
Everything in this world is undergoing a constant and unstoppable change. This is the absolute, immutable law of nature, governing not only the material world but also all realms of manifestation, including the spiritual. 'Everything in this world appears and disappears' - say the sages.
Similarly the transmissions of energy from the human mind as impulses, desires, emotions, ideologies, thoughts, resolutions, etc. These too are perpetually moving around the world. Mysteriously these energy waves travel around the world without the help of any tangible medium. (cf. rapid expansion of mystical, spiritual, religious visions and, political ideologies around the world in the past). Identical imaginations and ideas arise in many minds simultaneously. Ideologies of individuals get transformed into mass movements and revolutions. Mass thinking by groups of immoral persons pollutes the environment. A gentleman feels suffocated at such places. Conversely one feels relaxed at a place of healthy entertainment and peaceful at a place of worship.
The mindset of man is always in a state of flux. An individual pledged to continence throughout life till yesterday is found infatuated with sex today. The most hated enemy of yesterday becomes the most loved person of today. The person who sweats round the clock for earning more and more wealth, suddenly becomes a hermit. There are numerous examples of such transformations in the mindsets of individuals. Why? It is because constituents of mental energy as old thoughts and ideologies are being perpetually replaced by new ones.
