The Birkhoff-Maltsev problem asks for a characterization of those lattices each of which is isomorphic to the lattice L(K) of all subquasivarieties for some quasivariety K of algebraic systems. The current status of this problem, which is still open, is discussed. Various unsolved questions that are related to the Birkhoff-Maltsev problem are also considered, including ones that stem from the theory of propositional logics.

The genesis of time is explained in the spirit of constructivism combined with the activity approach to cognition. The cardinal temporal categories of present, past, and fu- ture are discussed in terms of action-thoughts understood as elementary units of activity whose structure is determined by linguistic semiosis. Husserl’s tripartite model of the phenomenology of time (prime perception, retention, protention) is applied to the ana- lysis of the subject’s experience of his actions. It is demonstrated that, while our lived present is (...) composed of the actually performed actions, our past and future are construc- ted by reflexive action-thoughts in the cognitive domain of language. It is emphasized that the construction of a temporal sequence that unites what is and what already or still is not, is possible only in linguistic semiosis. The analogy with Husserl’s tripartite structure of the time-consciousness flow helps elucidate the triad ‘present-past-future’ as an instance of the epistemological trap of language: ‘past’ and ‘future’ are mental const- ructs that belong to the present just as any other act of thinking. (shrink)

We present some equivalent conditions for a quasivariety \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal {K}}$$\end{document} of structures to be generated by a single structure. The first such condition, called the embedding property was found by A.I. Mal′tsev in [6]. It says that if \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf A}, {\bf B} \in \mathcal {K}}$$\end{document} are nontrivial, then there exists \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\bf C} \in \mathcal{K}}$$\end{document} (...) such that A and B are embeddable into C. One of our equivalent conditions states that the set of quasi-identities valid in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathcal{K}}$$\end{document} is closed under a certain Gentzen type rule which is due to J. Łoś and R. Suszko [5]. (shrink)