Theo's designers wanted to support meta-knowledge and meta-reasoning. Theo therefore has a highly-structured, highly uniform frame-based knowledge representation. Having a uniform representation simplifies both reading and writing to the knowledge base: all knowledge is represented the same so there is no problem converting between knowledge formats. This policy is somewhat constraining, though, because not all knowledge is represented in frame-based formats well.
Related principles that aids meta-reasoning are that all knowledge is globally accessible (glassbox knowledge) and uniformly accessible using a frame-based knowledge representation system.
Go to a discussion of this property for multiple architectures.