The article aims to think the specificity of lacanian concept of object a, distinguishing it from the object such as it is constructed by experimental sciences. At the same time, following Lacan indications that the a would be a metaphor of the knowledge object, one pursues in what sense the psychoanalysis object and the science object could be approached. The conclusion is their trace in common would be the invariability. The feature of invariability of the a will be considered in a double meaning: as a cause of desire and as surplus of enjoyment, in the attempt to find a dimension where both are articulated
object a; scientific object; cause of desire; surplus of enjoyment