Many people seem to agree that Arthur Samuel wrote or said in 1959 that machine learning is the "Field of study that gives computers the ability to learn without being explicitly programmed".
For example the quote is contained in this page, that one and in Andrew Ng's ML course. Several articles also contain this quote, and the reference is always the following article, which doesn't actually contain the quote.
Is there a reliable source? Or is this actually not a quote, but rather an interpretation of Samuel's article?