Despite these achievements, Aristotle's errors are thought by some, such as Peter Medawar, to have held back science considerably. Bertrand Russell notes that "almost every serious intellectual advance has had to begin with an attack on some Aristotelian doctrine". Russell also refers to Aristotle's "Ethics" as "repulsive", and calls his logic "as definitely antiquated as Ptolemaic astronomy". Russell says these errors make it difficult to do historical justice to Aristotle, until one remembers what an advance he made on his predecessors.