To start, consider a decision with several possibilities. Assume that the possible choices are known, and the decision-maker has a feel for their current state. Specifically, the decision-maker needs to have knowledge of the current and future availability of energy and time. Energy is here used to represent potential action. Time means unallocated time - "free time".
This metaphor is very simple: it is to view decision options as machines of happiness; the possibilities of a decision are viewed as machines which take energy and time as input, and output happiness. The efficiency and parameters of these machines are then the key factors in choosing among possibilities.
Although the typical concept of a machine requires a mechanical construct, this can be abstracted to a general concept of a device which transfers energy from one place or form to another. Under this definition, a knife is a simple machine - it transfers mechanical energy at the handle to a concentrated region along the cutting edge. An internal combustion engine is a simple machine - it transmutes chemical energy in the fuel to mechanical energy of the output shaft. This general concept of a device to transmit and transmute energy can be further expanded to include machines which are not physical - they could be a process or algorithm, or some complex reaction between physical entities. All of these concepts fit the definition of a machine, and this highly abstract definition is what is used here.
The actual internal workings of a machine are unimportant. All machines are viewed as "black boxes", where the only thing that is known is what output results from a known input. This view reduces the chance that decisions will be bogged down in examining the machines in detail, and tries to keep the decision-maker focused on the larger picture of comparing machines.
Given the abstract definition of a machine, the definition of a machine of happiness is easy: a machine which takes energy and time as input, and outputs happiness. Examples of these sorts of machines range from hobbies (and the tools used to pursue those hobbies) to friendships and other social interactions. Once the notion of a happiness machine is clear, the basis of decision-making is then a comparison of these machines.
The machine parameters that are of importance for decision making are the following: conversion efficiency and activation threshold. Conversion efficiency is a measure of how much happiness results from some given input of energy and time. A high conversion efficiency means that the machine generates large amounts of happiness for small inputs of time and/or energy. Conversion efficiency should be considered for time and energy separately, as some machines are very efficient in one and not the other.
Activation threshold is a measure of how much energy and time is required to simply activate or maintain the machine. Without a certain level of input, some machines will either produce nothing or cease to exist altogether. Hence, some relatively efficient machines can have high activation thresholds - social interactions are a good example. These machines require large energy and time inputs to maintain the machine, but the relative efficiency (once running) is still quite high.
The actual units of these parameters (efficiency and threshold) are arbitrary and unimportant. Since only relative amounts are of concern, as long as the units are consistent between machines, they can be anything. Hence, trying to attach units such as Joules to the energy input, or seconds to the time input, is silly and pointless.
Determining the machine parameters is not always an easy task. Some machines have well-defined parameters, such as many hobbies; the amount of happiness (tiny, little, lots, huge) can be directly correlated to the time and energy spent. Other machines are more difficult to gauge; social interaction machines are typically very difficult to assess. Both the efficiency and threshold are so variable between machine instances (different people) and over time that initial assessments are often incorrect. Experience and careful thought can reduce the uncertainty and increase the accuracy of any machine parameter assessment. Accept that many assessments will initially be in error and have to be revised at a later date.
Machine parameters are very individual, and the parameters of one decision-maker may be completely different from those of another. Hence, it is necessary for each decision-maker to determine the parameters individually. In some cases, it can be determined that the parameters are consistent across multiple decision-makers, but this is uncommon.
Once the efficiency and threshold of the various machines available in a decision are known, it is a simple matter to determine which machine to use - typically the machine which maximizes happiness subject to the available energy and time inputs is the preferred choice. It should be noted that the major constraint on decisions is the availability of energy and time to feed the machines, so the future availability of input must be considered in choosing the machine. An efficient machine with high threshold may be a poor choice if future input will be below the threshold. Similarly, a less-efficient machine with low threshold is a poor choice if future inputs will exceed the threshold of a high-efficiency, high-threshold machine.
Note that this metaphor applies to hobbies, material possessions, and inter-personal relationships. A consistent view of all these disparate concepts is afforded by this metaphor. This allows for quick, rational decisions of what machines to use, free from distracting differences between options. Hence, it is entirely possible for someone to realize that sufficient input is not available to fulfill the activation threshold of a serious inter-personal relationship. In that case, attempting to use that machine would be a poor choice since no happiness would result from the input of energy and time. A different person might realize that the efficiency of hobby machines are very low, and sufficient input is available for social machines, and hence would choose social machines over hobbies.
This metaphor can be further extended to outputs other than happiness. Since the ultimate goal of modern life is to maximize happiness over the lifetime, happiness is the natural output choice. How to maximize happiness is a topic beyond the scope of this article.