This essay examines the posthumanistic elements revealed in the memory-oblivion and memory-reconstruction processes portrayed by the main characters experiencing dementia in the short story 'Goodbye to a Thing' by South Korean novelist Cho Hae-jin and the play The Nature of Forgetting characterized as both drama and physical theater by the British theater company Theatre Re. The theatrical work and process of memory-oblivion illuminated in each of these stories metaphorically portray the characters’ loss of memory as human objectification, continuously juxtaposing the existence of human and nonhuman entities, namely, things. For the characters gradually forgetting their experiences, the memories that they recall are not merely formed through their own agency. From posthumanistic perspectives, the characters in the two works can be regarded as one participant within the assemblage process in which humans and nonhumans collectively engage. Through this assemblage, the oblivion of memory meditates between humans and all things, enabling humans to recognize the value and harmonious agency of all things, suggesting that such oblivion is not a tragic occurrence but rather a means of acknowledging the harmony and value of all things. It also encourages humans to participate more humbly and purely in the simultaneous moments of future mutual processes—that is, assemblage—more sincerely. This marks a turning point even in narratives dealing with dementia in existing literary works, allowing it to be seen not as a negative motif but as a new motif for contemplating the meaning of memory.