From Benjamin Graham & David Dodd’s Security Analysis:

The cardinal defect of instability may not be regarded, therefore, as menacing the long-range development of common stocks as a whole. It does indeed exert a powerful temporary effect upon all business through the variations of the economic cycle, and it has permanently adverse effects upon individual enterprises and single industries. But of these two dangers, the latter may be offset in part by careful selection and chiefly by wide diversification; the former may be guarded against by unvarying insistence upon the reasonableness of the price paid for each purchase.