For this to make sense we have to understand that mind creates thoughts and thoughts trigger the body to create hormones. When we have elevated thoughts, we create hormones that make us feel good, happy and peaceful and help the proper functioning of the body. On the polar opposite, when we have negative thoughts, bad thoughts or insecure thoughts we trigger our endocrine system to produce hormones that will make us feel negative emotions. In other words we begin to feel the way we're thinking, and thus our immune system will become weaker as a result.
Because the brain is in constant communication with the body, it's the brains job to know how the body is feeling. Emotions such as anger, frustration of fear trigger the brain to create more thoughts of the same frequency as our emotions. So we constantly think the way we're feeling and feel the way we're thinking.
Once we bring our conscious attention into our emotions, we can begin to transcend them, by shifting our focus on more elevated thoughts. When we find ourselves being fearful for instance, we must shift our focus on the polar opposite of fear which is courage. Know that where focus goes energy flows. Focusing on courage while breathing deeply will increase our vibratory frequency. Now the brain will start thinking more empowering thoughts and we will start a new cycle of thinking and feeling. Empowering emotions and thoughts will also increase our creativity, and as a result we will make better decisions in our life.
Of course this will fade away if the brain is hard wired into negativity. There is an identity behind our thoughts and the brain will hold to it as if we are threatening its existence, which in a way it's true. That's why we have to constantly become consciously aware of our emotions for a while, until we transcend each one of them.
And this is what our ancestors and sages meant by mental alchemy.