This comes from Andrew Roberts's magisterial biography of Lord Salisbury. In 1877 Salisbury, then Secretary of State for India became embroiled in the usual row between Viceroy and Secretary of State and as it was all too often the case, it was about Russian expansion in Central Asia and how Britain should respond. Lord Lytton, the Viceroy, was in favour of some action; Salisbury was considerably more cautious and less certain that Russia, despite the speed of her expansion, which he underestimated, was a real threat to India.
I think you listen too much to the soldiers. No lesson seems to be so deeply inculcated by the experience of life as that you should never trust experts. If you believe the doctors, nothing is wholesome: if you believe the theologians, nothing is innocent: if you believe the soldiers, nothing is safe. They all require to have their strong wine diluted by a very large admixture of insipid common sense.