Below, from the under-used, but excellent, textbook by Edward Tufte: Data Analysis for Politics and Policy (Prentice Hall: 1974)
One final point about the relationship between causal inference and statistical analysis. Statistical techniques do not solve any of the common-sense difficulties about making causal inferences. Such techniques may help organize or arrange the data so that the numbers speak more clearly to the question of ausality - but that is all statistical techniques can do. All the logical, theoretical, and empirical difficulties attendnant to establishing a causal relationship persist no matter what type of statistical analysis is applied. "There is," as Thurber moralized, "no safety in numbers, or in anything else."