Full Idea
Why not assume that God initially created the soul and body with so much ingenuity that, whilst each follows its own laws and properties and operations, all thing agree most beautifull among themselves? This is the 'hypothesis of concomitance'.
Gist of Idea
Assume that mind and body follow their own laws, but God has harmonised them
Source
Gottfried Leibniz (A Specimen of Discoveries [1686], p.80)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.80
A Reaction
They may be in beautifully planned harmony, but how do we know that they are in harmony? Presumably their actions must be compared, and God would even have to harmonise the comparison. Parallelism seems to imply epiphenomenalism or idealism.