Those who have already passed through the Gates of Death want to work with us on the physical world. This co-operation of theirs appears to be only a physical working with us, but everything physical is only an outer expression of the spirit.
The age of materialism has estranged man from the world of the dead; Spiritual Science must re-establish the friendship between them. The time must once more come, when we shall cease to make the work of the dead for the spiritualisation of the physical world impossible, by estranging ourselves from them. For the dead cannot take part with hands in the events of the physical, they cannot accomplish physical work in that direct way.
It would be foolish to believe that. The dead can work in a spiritual way. But to do so they need to have instruments placed at their disposal; they require the spiritual matter that lives here in the physical world. We are not merely human beings, we are also instruments, instruments for the spirits who have passed through the Gates of Death. As long as we are incarnated in physical bodies we use the pen or the hammer or the axe; when we are no longer incarnated in physical bodies the instruments we use are the human souls themselves.
Source: Rudolf Steiner – GA 175 – Cosmic and Human Metamorphoses – Lecture 5 – The human soul and the universe – Berlin, 6th March 1917
Translated by Harry Collison