This quotation is based on Dr Bach's philosophy of Unity between each individual's Soul (or Higher Self) and his mind and physical body. A balance between all three is essential for perfect health and happiness. If we follow the dictates of our own Soul by being true to ourselves and walking the path intended for us, not allowing others to influence or affect us by their own concerns and wishes, then all is well: we have joy, peace, health and happiness.
I recently had the pleasure of spending some time at The Dr Bach Centre in Oxfordshire where I am a Level 3 student. It is truly an oasis of peace and calm and I would urge you to stop by there if you can. The gardens, where many of Dr Bach's healing herbs grow today, are at their best just now and a wonderful place to spend some time in quiet contemplation.
Below is a link to The Twelve Healers: The Definitive Edition and there are other free downloads available on The Bach Centre website for you to enjoy.
https://www.bachcentre.com/new/education/books-and-reading-lists/free-downloads/