Engineered beings or weapons (in many cases conceptual entities are both) which exist only as concepts, and have no provable substance at all. This is a reasonably subtle idea, and several cultures involved in the War have had difficulty grasping it. As conceptual entities only seem to affect the minds of their victims, it's often said that the entities are 'made out of pure thought', but this is clearly inaccurate as thought itself isn't a substance. Although many people are determined to think of the entities as telepathic presences, or neurological parasites, or in some cases even “spirits”, in fact it's much more accurate to think of them as nothing more than hostile ideas. They exist by bypassing matter altogether, and instead giving themselves structure inside the meanings of things. For example, a victim affected by a conceptual entity might be reading a book and suddenly discover that there are messages in the text which shouldn't be there, communications from an entity which has (like any good idea) taken root in the invisible connection between the book and the reader's mind. The victim isn't hallucinating these messages. They have indeed been planted in the book, probably in exactly the same font and style as the rest of the text. But the entity hasn't changed the physical nature of the pages at all. Instead, the network of understanding which surrounds the reader - what Nevitz called “the topology of comprehension” - has been used to alter the meaning behind the book. In effect, the entity has wormed its wav into culture itself. The Celestis are, at least in part, a race of high-intensity conceptual entities. It's therefore not surprising that the Celestis are still the masters of conceptual engineering, and in recent years those Celestis who oppose the Homeworld have supplied the enemy forces with a whole host of specially-designed military concepts. The most blatant of these are, of course, the anarchitects. To some extent the Celestis' realm of Mictlan is an entire conceptual ecosystem, and is thought to be unique in this respect. — The Book of the War

For Doctor Who references, see:

• The Land of Fiction (old series story: The Mind Robber)
• Conceptual Space (new series episode: Space)
• Conceptual Bomb (Big Finish audio drama: The Light at the End)

Conceptual entities do not usually manifest in any physical way, as such they have no physical attributes and may never use them in Moves. However there are some exceptions - such as the Investigators sent by the Celestis as agents into the real universe. These entities have any attributes they wish.