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Psychology holds an exceptional position among the sciences. Yet even after 140 years as an independent discipline, psychology is still struggling with its most basic foundations. Its key phenomena, mind and behaviour, are poorly defined (and their definition instead often delegated to neuroscience or philosophy) while specific terms and constructs proliferate. A unified theoretical framework has not been developed and its categorisation as a ‘soft science’ ascribes to psychology a lower level of scientificity. The article traces these problems to the peculiarities of psychology’s study phenomena, their interrelations with and centrality to everyday knowledge and language (which may explain the proliferation and unclarity of terms and concepts), as well as to their complex relations with other study phenomena. It shows that adequate explorations of such diverse kinds of phenomena and their interrelations with the most elusive of all—immediate experience—inherently require a plurality of epistemologies, paradigms, theories, methodologies and methods that complement those developed for the natural sciences. Their systematic integration within just one discipline, made necessary by these phenomena’s joint emergence in the single individual as the basic unit of analysis, makes psychology in fact the hardest science of all. But Galtonian nomothetic methodology has turned much of today’s psychology into a science of populations rather than individuals, showing that blind adherence to natural-science principles has not advanced but impeded the development of psychology as a science. Finally, the article introduces paradigmatic frameworks that can provide solid foundations for conceptual integration and new developments.

Exploring human uniqueness encounters fundamental challenges because we can approach this endeavour only from within our uniquely human perspective. The intrinsic presumptions that this involves may entail two types of anthropocentric, ethnocentric, and egocentric biases, which can influence research on both epistemological and methodological levels. Their impact may be particularly pronounced if quests for the origins of human sociality are based only on our knowledge about humans. Tomasello's (2019) research demonstrates that the comparative study of humans and nonhuman species offers unique opportunities to explore forms of social cooperation, underlying cognitive and meta‐cognitive abilities as well as pathways in their ontological and (possible) phylogenetic development. It also shows that comparative approaches are essential to unravel the ways in which humans are indeed unique. But species comparisons are challenged by the need to consider inherent trade‐offs between achieving operational comparability in empirical studies and establishing ecological validity for the species compared—challenges, which analogously occur in comparisons across human cultures as well. This shows that comparative research can also contribute meaningfully to methodology development in psychology.

Rating scales are popular methods for generating quantitative data directly by persons rather than automated technologies. But scholars increasingly challenge their foundations. This article contributes epistemological and methodological analyses of the processes involved in person-generated quantification. They are crucial for measurement because data analyses can reveal information about study phenomena only if relevant properties were encoded systematically in the data. The Transdisciplinary Philosophy-of-Science Paradigm for Research on Individuals (TPS-Paradigm) is applied to explore psychological and social-science concepts of measurement and quantification, including representational measurement theory, psychometric theories and their precursors in psychophysics. These are compared to theories from metrology specifying object-dependence of measurement processes and subject-independence of outcomes as key criteria, which allow tracing data to the instances measured and the ways they were quantified. Separate histories notwithstanding, the article’s basic premise is that general principles of scientific measurement and quantification should apply to all sciences. It elaborates principles by which these metrological criteria can be implemented also in psychology and social sciences, while considering their research objects’ peculiarities. Application of these principles is illustrated by quantifications of individual-specific behaviors (‘personality’). The demands rating methods impose on data-generating persons are deconstructed and compared with the demands involved in other quantitative methods (e.g., ethological observations). These analyses highlight problematic requirements for raters. Rating methods sufficiently specify neither the empirical study phenomena nor the symbolic systems used as data nor rules of assignment between them. Instead, pronounced individual differences in raters’ interpretation and use of items and scales indicate considerable subjectivity in data generation. Together with recoding scale categories into numbers, this introduces a twofold break in the traceability of rating data, compromising interpretability of findings. These insights question common reliability and validity concepts for ratings and provide novel explanations for replicability problems. Specifically, rating methods standardize only data formats but not the actual data generation. Measurement requires data generation processes to be adapted to the study phenomena’s properties and the measurement-executing persons’ abilities and interpretations, rather than to numerical outcome formats facilitating statistical analyses. Researchers must finally investigate how people actually generate ratings to specify the representational systems underlying rating data.

The Transdisciplinary Philosophy-of-Science Paradigm for Research on Individuals (TPS-Paradigm) builds on established concepts, approaches, and methods from various disciplines that are systematically integrated into coherent philosophical, metatheoretical, and methodological frameworks and that are further developed and complemented by novel ones. The TPS-Paradigm is aimed at supporting scientists exploring individuals to tackle the particular challenges of their field and to make explicit and critically reflect on their philosophical presuppositions, metatheories, and methodologies. The metatheoretical definition of ‘personality’ as individual-specificity is elaborated, highlighting that ‘personality’ is conceived and studied differently because different paradigms focus on individual-specificity in different kinds of phenomena. Critical analyses of the field’s strong reliance on assessments methods reveal fallacies, circular explanations in ‘trait’ concepts, and the fact that taxonomic models of most of the phenomena in which individual-specificity is conceived as ‘personality’ still need to be developed. The TPS-Paradigm is therefore aimed at reviving and broadening the portfolio of methodologies that can and should be complementarily applied to comprehensively explore individuals and their ‘personality.’ Such broad paradigms are needed to enable transdisciplinary collaboration and research that embraces the complex reality of individuals and their ‘personality.’

Salt stress at different plant species may affect the reduction or increase in the content of dry matter, biogenic elements and specialized metabolites. The goal of this research was to determine the effect of salt stress on specialized metabolites accumulation and macroelements’ content of hydroponically grown rocket which belongs to the group of functional food. Salt stress was realized by adding NaCl (2.92, 5.27 and 7.62 g L-1) into the standard nutrient solution with EC-value 3 dS m-1, which showed increase in EC-values (5, 7 and 9 dS m-1), respectively. The trial was set up in a floating system according to a randomized block design with three replications. Salt stress had negative effect on dry matter and content of macroelements (N, P, K, Ca and Mg), while the amount of Cl and Na was increased linearly. Salt stress had positive effect on vitamin C and total phenols content. The highest concentration of NaCl resulted in the significantly highest vitamin C content (88.07 mg 100 g-1 fresh weight). The highest total phenols content (173.6 and 173.4 mg GAE 100 g-1 fresh weight), without significant difference, was achieved at rocket grown in nutrient solutions with EC 7 and 9 dS m-1. The equal content of total chlorophyll (0.9 and 0.8 mg g-1 fresh weight) was determined at nutrient solution with standard and the highest EC-values (3 and 9 dS m-1), respectively. Nutrient solutions with higher NaCl concentrations resulted in significantly lower content of total glucosinolates in rocket leaves than the standard nutrient solution, as well as with the lowest NaCl concentration.

Personality psychology is fragmented across heterogeneous subfields each focussing on particular aspects of individuals and from particular paradigmatic perspectives. Attempts for integration into overarching theories as that presented in the target article are therefore important. But the ideas proposed build on vague and often circular definitions of basic terms and concepts that hamper advancement and integration. My critique from philosophy-of-science perspectives pinpoints central problems and presents alternative concepts to help overcome them. A metatheoretical definition highlights the core ideas underlying common personality concepts and opens new avenues for conceptual integration.

Behaviour is central to many fields, but metatheoretical definitions specifying the most basic assumptions about what is considered behaviour and what is not are largely lacking. This transdisciplinary research explores the challenges in defining behaviour, highlighting anthropocentric biases and a frequent lack of differentiation from physiological and psychical phenomena. To meet these challenges, the article elaborates a metatheoretical definition of behaviour that is applicable across disciplines and that allows behaviours to be differentiated from other kinds of phenomena. This definition is used to explore the phenomena of language and to scrutinise whether and under what conditions language can be considered behaviour and why. The metatheoretical concept of two different levels of meaning conveyed in human language is introduced, highlighting that language inherently relies on behaviours and that the content of what-is-being-said, in and of itself, can constitute (interpersonal) behaviour under particular conditions. The analyses reveal the ways in which language meaningfully extends human's behavioural possibilities, pushing them far beyond anything enabled by non-language behaviours. These novel metatheoretical concepts can complement and expand on existing theories about behaviour and language and contribute a novel piece of theoretical explanation regarding the crucial role that language has played in human evolution.

Individuals are explored in various kinds of phenomena and contexts. But how can scientists compare individual variations across phenomena with heterogeneous properties that require different methods for their exploration? How can measurements of individual variations be made directly comparable between different studies, groups of individuals or even species? This research applies the Transdisciplinary Philosophy-of-Science Paradigm for Research on Individuals (TPS-Paradigm) to elaborate metatheoretical concepts and analytical methodologies for quantitative comparisons of individual variations within and across situations, groups and species using behavioural phenomena as examples. Established concepts from personality psychology, differential psychology and cross-cultural and cross-species research are systematically integrated into coherent frameworks and extended by adding concepts for comparing individual-specific variations (i.e., “personality”) between species. Basic principles for establishing the functional comparability of behavioural and situational categories are elaborated while considering that individuals from different groups and species often show different behaviours and encounter different situations and therefore cannot be studied with identical variables as is done in assessment-based research. Building on these principles, the chapter explores methodologies for the statistical analyses of the configurational comparability of constructs and of mean-level differences between groups and species. It highlights that situational properties are crucial for quantitative comparisons of individual variations. Fundamental differences between observational methods and assessment methods are explored, revealing serious limitations and fallacies inherent to comparisons of individuals on the basis of assessments. Implementations of the methodological principles and concepts presented are illustrated with behavioural data from four primate species (weeper capuchins, mandrills, toque macaques and rhesus macaques).\ud \ud This research was funded by a grant from the German Research Foundation (DFG; UH-249/1-1).

Animal researchers are increasingly interested in individual differences in behavior. Their interpretation as meaningful differences in behavioral strategies stable over time and across contexts, adaptive, heritable, and acted upon by natural selection has triggered new theoretical developments. However, the analytical approaches used to explore behavioral data still address population-level phenomena, and statistical methods suitable to analyze individual behavior are rarely applied. I discuss fundamental investigative principles and analytical approaches to explore whether, in what ways, and under which conditions individual behavioral differences are actually meaningful. I elaborate the meta-theoretical ideas underlying common theoretical concepts and integrate them into an overarching meta-theoretical and methodological framework. This unravels commonalities and differences, and shows that assumptions of analogy to concepts of human personality are not always warranted and that some theoretical developments may be based on methodological artifacts. Yet, my results also highlight possible directions for new theoretical developments in animal behavior research.