Semiosis is a kind of triadic action. Peirce (CP 5.472-3, c. 1906) explains that the difference between ‘dynamical, or dyadic, action’ and ‘intelligent, or triadic action’ is that the latter involves the use of means to an end. In dyadic action, A causes B, and if B later causes C as a separate event, we simply have another dyadic action intrinsically unrelated to the first. But in triadic action, A causes B because B causes C, and A will be unlikely to cause B unless B tends to be followed by C. Living systems have flexibility (and thus viability) because a variety of B-type actions (differing in details) can bring about C. Consciousness additionally confers the ability to choose which variant of B to employ in a given situation, enabling a measure of self-control.
All dynamical action, or action of brute force, physical or psychical, either takes place between two subjects,—whether they react equally upon each other, or one is agent and the other patient, entirely or partially,—or at any rate is a resultant of such actions between pairs. But by “semiosis” I mean, on the contrary, an action, or influence, which is, or involves, a cooperation of three subjects, such as a sign, its object, and its interpretant, this tri-relative influence not being in any way resolvable into actions between pairs.
EP2:411 (MS 318)
The actions of a life form, being intentional at some level of intelligence or complexity, are essentially triadic because they involve signs mediating between events and actions in such a way that the interpretant action (the ‘dynamic interpretant’) is determined by some purpose or habit, although it is also caused by some dyadic action. For that dyadic aspect of causality as opposed to the triadic, Peirce often uses the Aristotelian terminology of efficient cause as opposed to final cause. However, genuine triadic sign-action involves both kinds of causation. The dyadic action between object and sign is essential to the sign-function we call indexical, as Peirce goes on to explain:
For the acceleration of the pulse is a probable symptom of fever and the rise of the mercury in an ordinary thermometer or the bending of the double strip of metal in a metallic thermometer is an indication, or, to use the technical term, is an index, of an increase of atmospheric temperature, which, nevertheless, acts upon it in a purely brute and dyadic way. In these cases, however, a mental representation of the index is produced, which mental representation is called the immediate object of the sign; and this object does triadically produce the intended, or proper, effect of the sign strictly by means of another mental sign; and that this triadic character of the action is regarded as essential is shown by the fact that if the thermometer is dynamically connected with the heating and cooling apparatus, so as to check either effect, we do not, in ordinary parlance speak of there being any semeiosy, or action of a sign, but, on the contrary, say that there is an “automatic regulation,” an idea opposed, in our minds, to that of semeiosy.
CP 5.472-3 (MS 318)
The functioning of a thermostat is not considered semiosic because no mental action connects the dyadic action of the environment upon the thermometer with the dyadic action directly affecting the heating or cooling apparatus. But mental sign-action does occur when someone reads a thermometer and interprets the reading as an index of a fever, or of an overheated or underheated space. The actual response of the reader will then be intentional, i.e. mental, rather than automatic, and will thus be the completion of a triadic action. In the case of an index, though, the dyadic action of the object upon the sign is essential to the possibility of the sign conveying any information, or actually functioning as a sign. We might think of a thermometer which can be (but has not been) read as a “potential” sign, but as a sinsign, we assume that it is what it is whether anyone reads it or not, just as we assume that existing visible things remain what they are when nobody is looking at them. Likewise, we may call an uninterpreted index simply an ‘index.’ The same goes for a ‘symbol’ – which indeed must involve an index, for as Peirce told us (EP2:193), ‘every symbol must have, organically attached to it, its Indices of Reactions’; and the same goes for every argument, since every argument is a symbol.
The truth of a sign depends on the dyadic or real relation between the sign and its dynamic object. A true proposition must involve ‘action of brute force, physical or psychical,’ of the dynamic object upon the sign, so that the relation between the two is ‘real,’ i.e. surd – no sign can express or describe it. ‘Relations are either dicible or surd. For the only kind of relation that could be veritably described to a person who had no experience of it is a relation of reason. A relation of reason is not purely dyadic: it is a relation through a sign: that is why it is dicible’ (EP2:382-3).