<p>Both scientists and philosophers have increasingly focused on the prospect of moral neuroenhancement—the use of neurotechnologies or psychoactive substances to facilitate moral improvement. Recent scholarship distinguishes between two main approaches: direct moral neuroenhancement, which seeks to implant specific moral beliefs, motives, or behaviors, and indirect moral neuroenhancement, which aims to enhance capacities such as moral reasoning, conceptual understanding, or self-control, enabling individuals to arrive at better moral judgments through their own deliberation. Several philosophers have argued that the direct approach is ethically impermissible, as it risks undermining moral autonomy and suppressing valuable disagreement. The indirect approach, by contrast, is taken to avoid these concerns and is widely regarded as ethically permissible or even required. In this paper, we introduce a novel challenge for the project of moral neuroenhancement—the discernibility challenge: for any given intervention, it is exceedingly difficult, if not impossible, to determine whether it truly qualifies as indirect rather than direct. We demonstrate this by examining a range of experimental designs intended to discern between these two types of interventions, arguing that the evidence they produce is ultimately insufficient for this task. The discernibility challenge, we propose, exposes a profound epistemic uncertainty with notable implications for the ethical permissibility of moral neuroenhancement: if we cannot reliably distinguish permissible (indirect) from impermissible (direct) interventions, we lack a crucial epistemic basis for ethically justifying their use.</p>