It is well known that experts’ opinion and testimony take on a
decisive weight in judicial fact-finding, raising issues and perplexities that have long
been under scholarly scrutiny. In this paper I argue that expert’s opinions have a
much wider impact on legal decision-making. In particular, they may generate a
problem that I will call ‘the opacity of law’. A legal text, such as a statute or
regulation, becomes opaque if a legal authority is not able to grasp its full linguistic
content but is nevertheless in a position to use it, thanks to an expert’ opinion, in
legal decision-making. When this occurs, not only do experts contribute to factfinding
but also to determining the content of the law. In the paper I analyse the
linguistic and cognitive sources of this phenomenon, its characteristics and troublesome
consequences, and the different kinds of opacity that may affect legal
decision-making.