墨子 Mozi · Chapter 43
Canon Explanation, Part 2
Explanations of Argumentation Methods
止：彼以此其然也，說是其然也；我以此其不然也，疑是其然也。
Stopping an argument: The opponent uses a case to show that something is so, and explains why it is so. I use the same case to show it is not so, and cast doubt on whether it is so.
Explanations of Optical Phenomena
景：光至，景亡；若在，盡古息。景：二光夾一光，一光者景也。景：光之人煦若射。下者之人也高，高者之人也下。足敝下光，故成景於上；首敝上光，故成景於下。在遠近有端，與於光，故景障內也。景：日之光反燭人，則景在日與人之間。
Shadows: When light arrives, the shadow disappears; if the shadow remains, all past motion has ceased. Shadows: When two light sources flank one source, the single source produces the shadow. Shadows: Light strikes a person like an arrow shot. A person below appears high; a person above appears low. The feet block the lower light, so the image forms above; the head blocks the upper light, so the image forms below. Distance determines the point, and in relation to the light, the image is therefore inside the barrier. Shadows: When sunlight is reflected back onto a person, the shadow is between the sun and the person.
Notes
These explanations describe the pinhole camera effect (camera obscura): light passing through a small aperture produces an inverted image. The Mohist explanation that 'feet block lower light, forming an image above' and 'head blocks upper light, forming an image below' is a correct physical explanation of image inversion, making this one of the earliest recorded understandings of optics.
Explanations of Mechanical Principles
負：衡木，加重焉而不撓，極勝重也。右校交繩，無加焉而撓，極不勝重也。衡，加重於其一旁，必捶，權重相若也。相衡，則本短標長。兩加焉重相若，則標必下，標得權也。
The balance: A balance beam, when weight is added and it does not tilt, the fulcrum overcomes the weight. When the right arm's rope crosses and tilts without added weight, the fulcrum does not overcome the weight. When weight is added to one side of a balance, it necessarily drops -- weight and counterweight are comparable. At equilibrium, the shorter arm is at the base and the longer arm is the marker. When equal weights are added to both sides, the marker arm necessarily descends, because the marker arm has gained leverage.
Notes
This passage describes the lever principle and balance mechanics, showing an understanding of the relationship between weight and distance from the fulcrum. While less formally stated than Archimedes' lever principle, it demonstrates the same fundamental insight about mechanical advantage.