This paper examines certain theoretical constructs in law, legal theory, and communications theory that relate to secondary liability and its applicability to regulating online behavior. The central thesis of this paper is that a theoretical basis for ascribing legal responsibility to third parties can be based on that party's constitutive role in enabling illegal acts of others to produce social harm. Specifically, that the provision by an Internet service provider (ISP) of access to the enabling technical infrastructure - the network medium - in itself creates a responsibility base for mitigating social harm arising from the use of that infrastructure based on the effects of the mediation in furthering or contributing to the harm. The purpose of this paper is to explore certain theoretical constructs in order to develop an analytic framework that is potentially useful in delineating the appropriate scope, and under what circumstances and rationale, legal responsibility can be ascribed to ISPs for the actions of their users. Thus, in Part I we examine the theoretical basis for legal responsibility; in Part II, we review the existing doctrines of secondary liability as applied in particular contexts, (copyright, defamation, respondeat superior) in Part III, we suggest a performative standard for assessing ISP liability; in Part IV, we develop a preliminary approach for applying a constitutive standard for third party liability to ISPs; and, in Part V, we set out areas for further development. Applying the constitutive paradigm for legal responsibility, ISPs can be ascribed liability for providing access to the network and for their user's misconduct in so far as network access (or other services) give the conduct its effect. ISPs acquire subject-responsibility by virtue of their role in enabling the condition for user misconduct and object-responsibility for the online status of their user itself. These dual responsibility relationships provide a responsibility base for the harmful affects of the user's behavior. Additionally, the possession and exercise of the power to control harmful online conduct is itself a basis for assigning responsibility for mitigating the social harm. This paper explores third party liability under various theoretical constructs, including a constitutive paradigm for responsibility, existing legal doctrines of secondary liability, and a performative evaluation of effect. In Part IV we put forward a tentative framework for applying a performative standard for constitutive responsibility on ISPs for user misconduct based on negligence. However, this paper is not intended to be a definitive statement of these issues, but rather a preliminary research agenda delineating certain areas for further exploration. Finding or developing legal theories under which to impose responsibility on third parties without creating unwanted incentives for overdeterrence of desirable conduct is a difficult task. It is our position articulated here that constitutive liability for such third parties can be imposed directly based on an understanding of the performative affect of providing services that enable user misconduct to result in social harm.