A trigger device for setting a downhole tool is disclosed. The trigger device includes a retaining member that prevents the downhole tool from setting until it is properly positioned within the well. Regardless of the type of downhole tool or the type of trigger device, the retaining member includes a dissolvable material that dissolves when contacted by a solvent. The dissolvable material is preferably one that dissolves at a known rate so that the amount of time necessary for the downhole tool to set is pre-determined. Preferably, the solvent is a water-based or hydrocarbon-based drilling fluid or mud.