Beyond Illusions: On the Limitations of Perceiving Relational Properties1 Illusion?1.1 The underspecification problem (UP) 1.2 The Luther illusion1.3 Thesis: Illusionsm are not evidence of error but rather unmasking of error1.3.1 A note on so-called cognitive illusionsd2 Two case studies or how we deal with error2.1 Case study: Balancing as a relational property2.1.1 Methods detail2.1.2 Results2.2 Extending the case study: Comparing physics experts with non-experts 2.2.1 Methods detail2.2.2 Results and discussion2.3 The second case study: Visual cues to friction2.3.1 Friction as a relational property vs. surface roughness 2.3.2 Slipperiness Experiment: Visual cues to friction of familiar surfaces2.3.3 Methods detail2.3.4 Results2.3.5 Friction experiment with manipulated visual appearance2.3.6 Method detail2.3.7 Results3 Conclusion