indispensable evidence

His many sketchbooks provided a vast quantity of new and indispensable evidence as to what he had done and how he had gone about it.

Although it may have weakened their case, the prosecutors were right to play the clinic's videotapes, indispensable evidence on the issue of the spontaneity of the young witnesses.

For free blacks, who had only a precarious hold on freedom, "slave ownership was not simply an economic convenience but indispensable evidence of the free blacks' determination to break with their slave past and their silent acceptance - if not approval - of slavery."