This paper argues for a new index of performance based on {\em intimacy} 
for analysing and predicting human-computer interface and computer agent 
success.  Intimacy is defined as the subjective match between the behaviour 
of a device and the operation  of that device.  The main supposition 
of this paper is that a high degree of intimacy with a device enables 
communication of ideas and emotions effectively through the device as 
if it were an extension of the user.  Four main factors are considered 
which influence the level of intimacy: learning, responsiveness, attention 
and association with self or familiar objects.  Responsiveness includes: 
repeatability, adaptation and functionality.  Each of these influences the 
level of intimacy a user can achieve with a device as well as the speed 
of skill acquisition.  Techniques for measuring intimacy are included in 
the discussion.