 An alternative way of making sense of what genes do without the baggage of metaphors is to characterize it in causal language. C. Kenneth Waters drew on a manipulationist theory of causation that treated causes as manipulable difference-makers, where Waters’ insight was to add a distinction between “potential difference-makers” and “actual difference-makers” (Waters 2007). In carefully controlled genetic experiments such as Morgan’s classic work on fruit flies (see  section 1.1 above),  Waters pointed out that there were many potential difference-makers for a trait under investigation like eye color, but it was the actual genetic difference that was responsible for the actual difference in eye color. Waters claimed further that the spread of genetics throughout the life sciences was partly attributable to this feature of scientists being able to manipulate phenotypes by manipulating genes (entries:  molecular genetics;  causation and manipulability). As with the information metaphor, though, philosophical critics have been willing to accept Waters’ distinction between potential and actual difference-makers, but then challenged the idea that only genes acted as actual difference-makers (Griffiths and Stotz 2013; entry:  developmental biology).