Material objects give rise to physical happiness, while spiritual development gives rise to mental happiness. Since we experience both physical and mental happiness, we need both material and spiritual development. This is why, for our own good and that of society we need to balance material progress with inner development.
Because this age and the next age
Engender in the ditch,
No man can know a happy man
From any passing wretch,
If Folly link with Elegance
No man knows which is which.