Human beings are not very good at making decisions or planning. It is argued that a major reason for this lies in the essential logic that underlies decisionmaking discourse. Those with experience of decisionmaking will know that wild speculations and conflicting values confound the process, quite apart from logical confusions or mixing the general with the particular. This paper, the first of two, introduces a simple now-future logic which makes the concept of speculation well-defined. It is shown that speculations infect discourse with uncertainty out of all proportion to their frequency of occurrence. A ‘normative logic’ is introduced which allows arguments to be evaluated in terms of mixtures of good-bad-true-false. Decisions are sets of stated actions or intentions which are consistent with decision constraints as agreed by the decisiontakers. Decisionmaking authority is hierarchical and this is discussed in the context of the related, but distinct, hierarchy of set-theoretic aggregation of the particular into the general. Sixteen guidelines for decisionmakers are stated in terms of the structures defined. The forthcoming paper, which complements this, gives an extension of the ideas simply treated here with a discussion of strategic planning in terms of social time structurally defined as p-events.