Knot patterns first appeared in the third and fourth centuries AD, and can be seen in Roman floor mosaics of that time.
Interesting developments in the artistic use of interlaced knot patterns are found in Byzantine architecture and book illumination, Coptic art, Celtic art, Islamic art, Medieval Russian book illumination, Ethiopian art, and European architecture and book illumination. (Wikipedia).
The heart has long been used as a symbol to refer to the spiritual, emotional, moral, and in the past also intellectual core of a human being.
As the heart was once widely believed to be the seat of the human mind, the word heart continues to be used poetically to refer to the soul, and stylized depictions of hearts are extremely prevalent symbols representing love. (Wikipedia).