What is the difference between the heart (qalb), the soul (nafs), and the intellect/mind (fu'ad)?

The self (nafs) is applied to the soul, the blood, and the entire essence. It was named nafs due to its preciousness (nafasa) or from breathing (tanaffus). The verification is that the self is one entity with multiple attributes, including the 'at peace' (mutma'innah), the 'blaming' (lawwamah), and the 'inciting to evil' (ammarah). As for the heart (qalb) and the intellect/inner heart (fu'ad), it is well-known that they are one and the same, and their mention may be repeated with two different words for emphasis. It was also said: the fu'ad is the inner part of the qalb or its covering. The qalb was named qalb because of its constant changing (taqallub). And each of them may be applied to the other.