There is broad consensus (both scientific and philosophical) as to what a rigid body is in classical mechanics. The idea is that a rigid body is an undeformable body (in such a way that all undeformable bodies are rigid bodies). In this paper I show that, if this identification is accepted, there are therefore rigid bodies which are unstable. Instability here means that the evolution of certain rigid bodies, even when isolated from all external influence, may be such that their identity is not preserved over time. The result is followed by analyzing supertasks that are possible in infinite systems of rigid bodies. I propose that, if we wish to preserve our original intuitions regarding the necessary stability of rigid bodies, then the concept of rigid body must be clearly distinguished from that of undeformable body. I therefore put forward a new definition of rigid body. Only the concept of undeformable body is holistic (every connected part of an undeformable body is not always an undeformable body) and every connected part of a rigid body in this new sense is always a rigid body in this new sense. Finally, I briefly discuss the connection between this conceptual distinction and the dimensionality of space, thereby enabling it to be supported from a new and interesting perspective.

关于古典力学中的刚体，人们达成了广泛的共识（科学的和哲学的）。这个想法是，刚体是不变形的体（以所有不变形的体都是刚体的方式）。在本文中，我表明，如果接受此标识，则将存在不稳定的刚体。这里的不稳定性意味着某些刚体的演变，即使与所有外部影响隔离开来，也可能会随着时间的流逝而无法保持它们的身份。结果之后是分析在无限大刚体系统中可能发生的超级任务。我建议，如果我们希望保留关于刚体必要稳定性的原始直觉，那么就必须将刚体的概念与不变形体的概念区分开来。因此，我提出了刚体的新定义。只有不变形体的概念才是整体的（不变形体的每个连接部分都不总是变形体），而在这种新的意义上，刚体的每个连接部分在这个新的意义上始终都是刚体。最后，我简要讨论了这种概念上的区别与空间的维数之间的联系，从而使它能够从一个新的有趣的角度得到支持。