Animal welfare is an important concern in modern society. The most common ethical underpinning
of animal welfare is the concept of sentience. However, there is no agreement yet on the
definition of sentience and on which features are essential for a species to be classified as sentient.
Unsurprisingly, hot debates flare up periodically about whether a certain species could be considered
as sentient and thus on whether its welfare should be granted. In the present paper, we
outline the repetitive tendency of such debates, using fish and arthropods as an example. Up to
now, these debates tend to end with the vast majority of researchers either recognising sentience
in the target species or advising the use of the precautionary principle and thus tentatively act as
if the species is sentient in order to take decisions regarding its welfare status. The debate then
usually moves to a species progressively less similar to humans and the cycle of the ‘sliding scale’
begins anew. In view of this tendency, we discuss whether it would be advisable to reject the idea
of a sliding scale when welfare relevant decisions are at stake.
