Mental image vividness and aphantasia judgements are fundamentally compromised by overlooked definition problems.

Vividness judgements are intended to capture variance in the axis that uniquely separates the experience of a mental image from other unambiguously sensory perceptual experiences, and from a completely non-sensory experience of one's thoughts (aphantasia). Yet, despite being historically criticised as confounded, vividness judgements remain ubiquitous and aphantasia studies are becoming increasingly common. This implies that these metrics are still assumed to be valid and interpretable for their intended use, or that aphantasia judgements have dodged the issues affecting vividness judgements generally. This article argues that there is no logical basis for these assumptions. The hypothetical axis that singularly distinguishes a mental image from other perceptual experiences has eluded definition for centuries. While this axis remains undefined, variance along it cannot be validly measured, and any purported index of it is logically impossible to validate by correlation to another process. This fundamental problem is continuously overlooked and contradicts recent claims that aphantasia can be validly and objectively identified. Arguments are discussed suggesting that even if vividness judgements capture real variance, it is not the right variance, that vividness and aphantasia indices are circularly validated, that criterion confounds can explain common findings, that such confounds are real and pervasive in the community, and that there are legitimate reasons to doubt even highly confident imagery self-report. These issues likely stem from the widespread use of prohibitively ambiguous or philosophically unjustified references for what type of experience constitutes typical mental imagery. Better candidate references, and how they can correct current confounds, are discussed.